If you have one of the iMacs listed below, and you are replacing the optical drive with an Data Doubler and a 2.5" SATA drive, note that there is a thermal sensor about the size of a dime which is attached to the optical drive. It can be removed, then attached later to the new 2.5 inch drive. Be sure you do this before installing the drive into the Data Doubler. The plastic shield on the Data Doubler will prevent you from placing the sensor once the drive has been affixed to the Data Doubler. The shield is designed to deflect the airflow from the fan over to the GPU and/or other components — do not remove it.
• iMac early 2009 (20” or 24”), model iMac9,1*
• iMac late 2009 (21.5” or 27”), model iMac10,1*
• iMac late 2009 (27”), model Mac11,1*
• iMac mid 2010 (21.5”), model iMac11,2
• iMac mid 2010 (27”), model iMac11,3
• iMac mid 2011 (21.5”), model iMac12,1
• iMac mid 2011 (27”), model iMac 12,2
* The SATA controller used in iMac 2009 models (models iMac9,1; iMac10,1; iMac11,1) has a known limitation when using SATA 6.0Gb/s hard drives and solid state drives (SSDs). If a 6.0Gb/s drive is installed, the computer will limit the drive to 1.5Gb/s. Therefore it is recommended that only SATA 3.0Gb/s drives be used.
The picture below provides a visual reference to the sensor (black square and the plastic shield).