Desmo

I installed a 256GB blade SSD from a 2012 rMBP. Set up the fusion drive with the stock 1TB drive for 1.25TB total. It works perfectly, no abnormal fan spinning or anything.

The iMac is VERY fast now because the first 256GB of used space reside on the SSD, and if you stay under the size of the SSD, ALL data resides on it. Verified by running several tests...

Quote from Patrick Mast:

Be careful. There are 2 LONG screws and 2 short ones. Mark which got out where as the short ones do not fit in the long wholes.

I found one long screw. It goes in the corner right next to the fan.

Also, I would recommend taking the audio cable off nearest to the cd drive and not on the audio board. It's easier to see if it is lined up when reconnecting it.