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Thunderbolt on the M1 Mac mini – When 2 Actually Does Equal 4

Why I need four Thunderbolt ports

I have read a lot of complaints about the new M1 based Macs only having two Thunderbolt ports. Having only two ports doesn’t seem like enough for professional use. Both my 2019 16 inch MacBook Pro and my 2018 Mac mini have four Thunderbolt ports, and I can’t imagine using a Mac with only two.

My desire for more than two ports comes from using bus-powered devices like the OWC Envoy Pro EX and the OWC Thunderbolt 3 10G Ethernet Adapter. Both of them have only one Thunderbolt connection, so they must either be connected to a dedicated Thunderbolt port on my Mac or be at the end of a chain of devices.

On Intel Macs, each pair of Thunderbolt ports is actually one bus

My other concern is that both my Macs only really have two Thunderbolt buses. Each pair of Thunderbolt ports on my Intel Mac is connected to a single bus powered by a dedicated controller chip. Since the two ports in a pair are connected to one bus, they share the Thunderbolt bandwidth. The total throughput of both ports together on a single bus is limited to 2,800 MB/sec.

Infographic showing Thunderbolt controllers on a MacBook Pro
On a MacBook Pro, two Thunderbolt ports share one bus (Thunderbolt controller)

You can confirm the number of Thunderbolt buses in your Mac by viewing the System Report window (select About This Mac in the Apple menu and then click the System Report… button). Then click on Thunderbolt in the column on the left. Here is what I see on my 2019 16 inch MacBook Pro, showing the two Thunderbolt buses. (4 ports = 2 pairs of ports = 2 Thunderbolt buses)

2019 16-inch MacBook Pro System Report
2019 16-inch MacBook Pro System Report

Things are different on the M1 Macs

We received our first M1 Mac last week, and I have been using it ever since to debug and test the ARM version of the SoftRAID driver. The first thing I noticed was that the System Report indicated that there were two Thunderbolt buses, just like my 2019 16 inch Mac Book Pro.

2020 M1 mac mini System Report
2020 M1 mac mini System Report

Wow, that’s unlike any other Thunderbolt-equipped Mac I’ve ever used. System Report indicates that there are two Thunderbolt buses, one for each of the Thunderbolt ports!! This means that each port has its own dedicated bus and doesn’t have to share its bandwidth with any other port. Each one will have 2,800 MB/sec all to itself. I was a bit skeptical and wanted to test this to be certain.

Infographic showing Thunderbolt controllers on an M1 Mac mini
On an M1 mac mini the two Thunderbolt ports each have their own bus (Thunderbolt controller)

Testing the bandwidth on the two Thunderbolt ports of the M1 Macs

I used the AJA System Test Lite application, a file I/O benchmarking application, to test whether these two ports were indeed connected on separate Thunderbolt buses. I set up two OWC ThunderBlades and connected each to a separate Thunderbolt port on the M1 Mac mini. Each ThunderBlade contains 4 NVMe SSDs, so I created a stripe (RAID 0) volume using all eight blades to test performance. If the two Thunderbolt ports share bandwidth, I will not be able to read or write faster than 2,800 MB/sec, the limit of a single Thunderbolt bus. If the two ports are indeed on separate Thunderbolt buses, I will see faster performance.

M1 Mac mini AJA Performance Test Results
M1 Mac mini AJA Performance Test Results

The results using AJA System Test show that the 2 Thunderbolt ports of the M1 Mac are indeed on separate Thunderbolt buses as I get over 3,500 MB/sec reading from my stripe volume.

What about only being able to connect two Thunderbolt devices to the M1 Macs?

I may have the same bandwidth from two Thunderbolt ports on the M1 Macs as I have on the Intel Macs, but I still can only connect two devices directly to this new Mac. Isn’t this going to be a problem? It turns out that Thunderbolt 4 saves us here.

The move from Thunderbolt 3 to Thunderbolt 4

The M1 Macs were the first Macs with support for the Thunderbolt hub feature found in Thunderbolt 4. Thunderbolt 4 requires computer makers to implement a whole set of features, most of which were optional in Thunderbolt 3. These features have actually been found in almost all Macs that Apple has shipped in the past four years, so the move to Thunderbolt 4 isn’t a huge change for Mac users.

These features include 40 Gb/sec Thunderbolt ports, support for charging laptops over Thunderbolt, and protection from malicious hardware that might try and snoop computer memory over Thunderbolt. The one feature in Thunderbolt 4 new to the Mac is the support of Thunderbolt Hubs.

Note: With the subsequent release of Big Sur 11.1, Intel Macs gained support for Thunderbolt hubbing.

What is a Thunderbolt Hub?

A Thunderbolt hub, for instance, the OWC Thunderbolt Hub, gives you more ports to connect Thunderbolt devices. In the same way that Intel Macs have two Thunderbolt ports connected to a single Thunderbolt bus, a Thunderbolt Hub allows you to have three Thunderbolt ports connected to a single Thunderbolt port on a Mac. 

In both cases, you can connect a chain of devices to each of the downstream Thunderbolt ports (either the two ports on your Intel Mac or the three ports on a Thunderbolt Hub). This allows you to connect more devices that only have a single Thunderbolt connection and improves stability by separating storage devices from other less critical devices. (I usually suggest that displays being connected over Thunderbolt not be combined on the same chain as storage devices to improve the reliability of your mounted volumes.)

So with an M1 Mac and a single Thunderbolt Hub, I can have the stability and usability which comes from four Thunderbolt ports currently found in my Intel Macs and also have the high bandwidth which comes with two separate Thunderbolt buses. Combine that with the lack of fan noise and better battery life, and the move to an M1 Mac now looks very enticing.

Now, if only I can persuade my boss to get me a new M1 MacBook Pro to use as my development machine…

the authorOWC Tim
Vice President of Software Engineering, Mac
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  • I’m excited about these findings, but I’m confused about Thunderbolt 4. As fr as I know, Apple have state clearly that the M1 Minis do not have Thunderbolt 4. You, however, state “The M1 Macs were the first Macs with support for the Thunderbolt hub feature found in Thunderbolt 4. Thunderbolt 4 requires computer makers to implement a whole set of features, most of which were optional in Thunderbolt 3. These features have actually been found in almost all Macs that Apple has shipped in the past four years, so the move to Thunderbolt 4 isn’t a huge change for Mac users.”

    Does this mean the Minis still have 3 but with most of the advancements of 4? Did Apple tell us wrong? So on and so forth.

    Thanks in advance!

    • The M1 Macs don’t have Thunderbolt 4 because they don’t support 2 4K monitors – a requirement for Thunderbolt 4. I think that is the only Thunderbolt 4 feature they are missing.

      So since these Macs don’t have all of the features required for Thunderbolt 4 certification, they can’t be labeled Thunderbolt 4. Once again, Apple is going its own way rather than following the standard.


  • This is the biggest load of bullcrap I have heard, having only 2 physical port is so limiting. The intel mac’s also have 2 bus you still have full bandwidth when you only plug in one device pre bus. Furthermore, when don’t need the bandwidth and you need more device you can plug it in giving you double the flexibility.

    • I agree but then again, but Apple frequently removes truly useful functionality from its products. We lost the 3.5 mm headphone jack on iPhones, a world wide standard, as well as the USB-A port on laptops which everyone uses for thumb drives. Every portable electronic device I now own recharges via USB-C, except my iPhone.

      Maybe, like with the 2013 MacPro, they will actually admit they made a mistake and re-introduce those features.


    • I guess the point of this article is that the M1 Macs are better in regards to total PCIe bandwidth than the Intel Macs that have the most limited Thunderbolt 3 implementations (any non-Ice Lake based Intel Mac that has a single Thunderbolt bus).

      For PCIe bandwidth (ordered from low to high) (I don’t have all the numbers for ordering those in group 3 and 4 which have similar total PCIe bandwidth):
      1) Intel Mac with single discrete Thunderbolt 3 controller (PCIe 3.0 x4)
      2) Intel Mac with two discrete Thunderbolt 3 controllers (PCIe 3.0 x4 and x2)
      3) M1 Mac (integrated in CPU, similar to PCIe 3.0 x8) (two buses, one port per bus)
      4a) Intel Mac with two discrete Thunderbolt 3 controllers (PCIe 3.0 x4 and x4)
      4b) Ice Lake Mac (integrated in CPU, similar to PCIe 3.0 x8) (you can get max bandwidth from any two of the four ports)

      Usually max bandwidth is obtained by connecting to different Thunderbolt buses.
      You can add displays to one port of a bus without reducing PCIe bandwidth of the other port of the bus. #4 is better than #3 because it has two ports to connect displays and two other ports for data. A single 4K display (16 Gbps) doesn’t affect PCIe bandwidth much though (or 6K with DSC).

      For DisplayPort bandwidth (ordered from low to high):
      1) M1 Mac (HBR3 or 2xHBR2 but only for a tiled display)
      2) single Alpine Ridge (2xHBR2) (M1 Mac is better if you need HBR3 or DSC)
      3) single Titan Ridge (2xHBR3)
      4) Ice lake (2xHBR3) (better than #3 because there are four ports total; better than #5 if you need HBR3 or DSC)
      5) dual Alpine Ridge (4xHBR2)
      6) dual Titan Ridge (4xHBR3)

      Assume Link Width is four lanes.
      HBR3 is 150% of HBR2. HBR is 50% of HBR2.
      2xHBR2 or HBR3+HBR on a single port are possible.
      Only 2 DisplayPort connections per bus.
      2xHBR3 on a single port is possible only for direct connect Apple Pro Display XDR with GPUs that don’t support DSC (6K is tiled in this case, and 3008×3384 does not require entire HBR3 bandwidth so two streams of that can be transmitted by Thunderbolt 3).
      Some Macs with Titan Ridge use Intel Graphics so they are limited to HBR2 like Alpine Ridge.
      All Intel Graphics including Ice Lake are limited to 3 displays total (internal display or HDMI port counts as 1)

      The M1 Mac has only two ports, but you can turn two into six with a couple hubs or chain Thunderbolt devices together. M1 Mac is good for PCIe bandwidth if you don’t connect a display to the Thunderbolt ports. In any case, whether you have an M1 Mac or an Intel Mac, you should balance the bandwidth used by each Thunderbolt bus and port. 40 Gbps (PCIe + DisplayPort) per port, 24 Gbps PCIe per bus, 2 DisplayPorts per bus.

  • Great article. I have a new Mac Mini M1 – I have a 10 GB adapter plugged into one of my TB ports connected to my Synology NAS. Works great. The Second TB Port on My Mac Mini is plugged into a Universal Audio interface using a TB 2 to TB 3 convertor and it also works great.

    My problem is that I want to use 2 monitors. I have tried an Anker convertor but it does not work. I also tried the Apple TB to HDMI, USBC adapter but no luck.

    I assume the new OWC THUNDERBOLT HUB will work.

    The Universal Audio interface does have two TB 2 ports to allow you to daisy chain. I wonder if I can make that work?

    Any thoughts? Also when is the OWC Thunderbolt Hub coming out?

    • Hello James. The Hub is slowly starting to ship here in February! This should allow you to connect 2 displays (the M1 macMini is the only M1 machine that allows for 2 external displays, so it’s definitely doable!). Our Thunderbolt 3 Dock would work as well.

      • I’m pretty confused here: you say “Our Thunderbolt 3 Dock would work as well”: is the Thunderbolt 3 Dock allowing to have two (2) monitors working on the Mac mini M1 or not?
        Before it was said it cannot: only way is one (1) on Thunderbolt and one (1) on HDMI.

        • Yes, the Thunderbolt 3 Dock would allow it. One would run via the second Thunderbolt 3 port on the dock, the other would run through the Mini DisplayPort. You can use adapters into those ports, to adapt whatever displays you are using.

          • Has this been tested? Did Apple make a change in Big Sur recently to support this on M1 Macs? I don’t think so.

            A Thunderbolt device can only connect two displays if the host allows it. The M1 Macs do not allow two displays connected to the Thunderbolt ports or a Thunderbolt port. A Thunderbolt dock is not going to change that (unless the dock uses DisplayLink or MST but OWC doesn’t use DisplayLink in its docks and macOS doesn’t support MST for multiple displays).

            • You are correct, I mis-spoke, my apologies. You can use the dock or hub to connect a single display, with a second running through the HDMI port on the macMini itself. My apologies for the confusing answer!

    • The M1 Mac mini can connect two displays. One to the HDMI 2.0 port and another to a Thunderbolt port. Additional displays beyond those two can be added using DisplayLink (video compressed over USB to a display). Visit the DisplayLink website for more info.

      A Thunderbolt hub or dock does not increase the number of displays that can be supported. A Thunderbolt hub can increase the number of Thunderbolt devices that can be connected (especially if you’re connecting Thunderbolt devices that don’t have a second Thunderbolt port). You can use the Thunderbolt hub to connect all your devices (including a display) to a single Thunderbolt port of the M1 Mac but for a single Thunderbolt port, you should make sure that the data bandwidth doesn’t exceed 22 Gbps and that the data plus display bandwidth doesn’t exceed 40 Gbps – otherwise you should move a device to the other Thunderbolt port.

      I’m not sure what an Anker converter or a Apple TB to HDMI, USBC adapter are. Maybe you mean the Apple USB-C Digital AV Multiport Adapter. The adapter should work with the M1 Mac if you are not exceeding any of the limits described already. If your display supports DisplayPort then you should use a USB-C to DisplayPort cable because DisplayPort has more bandwidth than HDMI.

      When connecting Thunderbolt devices together, the faster Thunderbolt 3 devices should be connected to the Mac first. Only Thunderbolt 2 devices should be connected after a Thunderbolt 2 device. A DisplayPort 1.2 display (4K 60Hz) can be connected to the downstream Thunderbolt port of a Thunderbolt 2 device but you should make sure the data plus display bandwidth in the Thunderbolt 2 chain does not exceed 20 Gbps.

  • That’s interesting because I am unable to connect 2 separate simple NVMe drives at the same time on the 2 separate TB 4 buses on a M1 Air. One is in a TB 3 Wavlink enclosure and another in a Fledgling TB 3 enclosure.
    The message I get is that I need to disconnect one for the other to work.

    • I notice that both of the enclosures you are using are bus powered. They are also both available without blades. If you added your own blades and are using larger blades (2 TB or larger), there is a good chance that the two devices together are drawing more power than your MacBook Air can supply over Thunderbolt.

      A simple test of this would be to plug in each device by itself. If they both work correctly, by themselves, the power delivery limit is most likely the cause.

      You can fix this by connecting at least one of these devices using a Thunderbolt dock or hub. We make several which should work (https://eshop.macsales.com/shop/docks).

      You will need a powered dock with at least 3 USB-C connectors.


    • Yep, and it doesn’t matter what brand/type it is. I’ve tried several different combinations of enclosures/SSDs, consulted with senior Apple engineers, and tried multiple M1 MacBooks. The verdict is that the M1 MacBook simply doesn’t supply enough power to the Thunderbolt ports to drive more than one Thunderbolt device.

      I’m amazed that the press seems to be giving Apple a pass on this; this seems like a bigger scandal than “AntennaGate” or “BendGate”. They claimed these computers had “two” Thunderbolt ports, but they effectively only have one. Originally Apple even claimed that these were TB4 but at least they seemed to have walked that back.

      I would not have bought mine if I had known this, and by the time Apple support stopped jerking me around and admitted the computers simply can’t do it, the return period was over.

  • Thank you for this interesting article, and maybe its replies.
    I’m still wondering how things work: apparently you can have two displays running on the Mac mini M1 where one is attached to the Thunderbolt and one to the HDMI.
    That would mean the “graphics card” is capable of serving two displays.
    It has been ‘discovered’ the two Thunderbolt connections actually have separate channels: can someone explain why I cannot run two (Thunderbolt) (D/d)isplays on the two thunderbolt connections?

    I have tried this, I have tried to connect the HDMI outlet of the Mac mini with the thunderbolt to HDMI adaptor (didn’t work, guess HDMI or adaptor is one direction only?).

    Question now is will the Thunderbolt 3 Dock be able to steer the two Thunderbolt Displays on one (1) thunderbolt connection on the Mac mini M1?

    • The DisplayPort connection used by the HDMI port of the M1 Mac mini (or the M1 MacBook Air or Pro’s built-in display) doesn’t have a switch that would allow it to be used by the Thunderbolt buses (same for Intel Mac mini). In contrast, the DisplayPort connection used by the HDMI port of a W5700X MPX module of a Mac Pro does have a switch (called a MUX on page 14 of https://www.apple.com/mac-pro/pdf/Mac_Pro_White_Paper_Feb_2020.pdf )

      The two DisplayPort connections of a Thunderbolt port of a M1 Mac can both be used only for a dual link SST tiled display such as the Thunderbolt 3 LG UltraFine 5K or the dual-cable Dell UP2715K and similar displays. If a single link SST display is connected (4K or 6K with DSC), then the second DisplayPort connection is unused.

      The M1 Macs are using a graphics system similar to iOS devices. To support dual link SST displays, I guess they have a kind of meta display layer on top of the lower physical connection layer. In the display layer, you can have only two displays. But in the physical layer, a display may use two DisplayPort connections.

      Or maybe Apple got tired of explaining why you can connect two 4K displays to a Thunderbolt bus but only one 5K display unless the 5K display is a single link SST display. Now, they can just say one display, period.

      USB-C to HDMI 2.0 adapters are one way. There does exist HDMI 2.0 to USB-C adapters, but the Thunderbolt ports are for output only. The DisplayPort inputs to the Thunderbolt bus are inside the M1 Mac on the printed circuit board (PCB) so they cannot be easily modified. But modern Thunderbolt displays can accept non-Thunderbolt USB-C input so you can just use a HDMI 2.0 to USB-C adapter to drive a display like the LG UltraFine 4K at 4K 60Hz 8bpc RGB (HDMI 2.0 has less bandwidth than DisplayPort 1.2 so it cannot do 10bpc without chroma sub sampling).

      The limitation of one display total (either dual link SST or single link SST) to the Thunderbolt ports of an M1 Mac extends to any Thunderbolt docks connected to the M1 Mac. The max number of displays is one no matter how many Thunderbolt docks are connected.

      Future Apple Silicon Macs may allow more displays (hints of this already exists in the IO Registry of the M1 Macs). Who knows if Apple will ever add support for eGPUs?

      • Excellent reply on my question(s)!
        Thanks so much, it explains a lot.

        And I still think it is silly the two Thunderbolt ports on a Mac cannot connect to two displays via Thunderbolt (that is, if the graphics card is capable of doing so).

        Again, thanks!

    • Thanks for reply.
      Yes they work individually
      One has a 1TB Samsung 970 NVMe.
      The other an Intel 660.
      Interestingly the Intel has seriously degraded performance.
      Used to be around 1500 on Blackmagic, now down to 600.
      I’m waiting for your 3 port TB4 hub to ship.
      I already have the TB 3 dock,

  • Wouldn’t it be great to have a version with a 10GbE port on the TB hub as well? Because that’s what we actually miss on the m1. So, having to use 2 adaptor boxes just to have enough ports an 10GbE is so annoying. And wouldn’t it reduce the cost for OWC and the customer?

      • I know. I bought it 2 years ago when it was from Akitio. But it’s only TB3, has a super big external power supply with a stiff cable and also a very annoying fan. You wouldn’t like to have it on your desk. And you definitely wouldn’t like to have it with your MacBook. So what I’m looking for is a small fanless TB Hub with an integrated 10 GbE.
        I’m based in film production and have a lot to do with photographers, too.
        This is what everybody wants right now.
        If you have further questions please contact me. Thanks!

    • Yes, it is very similar to the M1 MacBook Pro. The processor is just thermally constrained because there is no fan. This means the CPU can’t run as fast and throughput to a volume connected via Thunderbolt is a little slower.