Thanks Hanno for this hint!
It should be noted here that on the SATA side of the cable the wires '''1''' and '''4''' have to be shorted (and '''NOT''' wire 2 and 7). This is much easier then Apple's official solution where you have to take the iMac almost completely apart.
For better understanding I have added below some (low resolution) photos. (Sorry for the bad quality).
'''CAUTION!!!''' This modification is '''ONLY VALID''' when you switch to a '''solid-state drive (SSD)'''.
'''DO NOT''' use this mod to connect a normal harddisk drive! This will most likely result in massive overheat of your iMac.
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