I had a friend who had bought, 2nd hand, a mid 2011, 27" iMac for his son and wanted to install an SSD into it to move the OS onto.
The repair went well up until the part after removing the optical drive. It was at this point that I started to get suspicious (whereas in reality I should have spotted the problem much sooner).
The iMac already had an SSD installed, in fact it was a factory fitted one, furthermore, that was the only drive in it as it did not have an HDD (that was the bit I should have spotted sooner).
So at this point I rang my friend who seemed as confused as I was as he was under the impression that it did have an HDD - even though he had partitioned the drive and installed BootCamp onto the partition. Even though the SSD was only 250GB, he didn't 'click' that it wasn't an HDD.
OK, so he decided to go ahead and get me to install the additional SSD, which then led me to the next problem that I discovered after removing the logic board, the SATA ports were both already in use, 1 by the SSD and 2 by the Optical Drive, so the decision had to be made as to what to sacrifice to be able to fit the 2nd SSD in. My friend decided that the Optical Drive was superfluous, so I removed the cable and proceeded with the SSD cable and drive install. I decided to place the SSD in the space that the HDD would have been.
All continued to go well - the Logic Board was difficult to get back in, but I finally managed, did a quick check that all the cables were clear and secured the Logic Board back with its screws.
I then go to reconnect the cables only to discover that the first cable - the power button connector - was nowhere to be seen. Frustratingly I undid all the screws again, removed the Logic Board, found the cable, secured it with sellotape t the side, repositioned the Logic Board and replaced the screws for the 2nd time. The rest of the replacement of cables went well.
I decided to put the Optical Drive back in, even though it would be non-functioning as I wanted to ensure the Thermal Sensor cable was connected to hopefully prevent any fan speed issues.
Well I finally got it all back together, booted up and -no fan noise, so yes for that, and the SSD was recognised and asked to be initialised which I did via Disk Utility.
So my friend was very happy as he now had more storage space for his son's computer.
A couple of tips:
1. Maybe boot the computer first to see what System Info tells you, before diving in and taking the word of the customer as to specs.
2. When disconnecting the cables, maybe tape them back out of the way onto the frame/chassis so they don't get caught under the Logic Board.
3. Finally go back through the guide to check that you have all the cables accounted for before securing the Logic Board back down.