I purposely sought out a pre-welded-body 27" iMac on eBay with the intent of upgrading everything inside as a hobby. The RAM going up to 32GB was easy, and when timed right, not that expensive. In hindsight, not that great of an investment. I bought a new/used (guy won it at an office party, and didn't want to go Mac) and I was able to get AppleCare on it. I took it to an Apple store and they not only changed out the recalled AMD video card, but replaced the logic board, which they said functioned, but failed several tests and couldn't explain why. Seemed odd, but no cost to me.
What happened next, well, it could have gone better.
OWC and iFixit are two great sources of not only information, but pieces and parts.
I had put an SSD in my 2011 MBP, and things went well. I even used TRIM Enabler successfully with Yosemite in that machine, and tested it with and without, eventually realizing that using a sledgehammer to crack a nut was not necessary. (Side note: Removing stock thermal paste and replacing with the popular alternative helped a little, but not much. No fails, yet.)
I thought putting an SSD in the iMac would be just as easy. Installing... not that bad. Getting it to run worry-free, not so much.
Opening the case and removing screws... watch the videos, follow the steps (in this case, preferred OWC's guidance), and take your time.
The conventional place to add an SSD is behind the CD/DVD drive. It literally sticks there with double-stick foam tape. I found the problem with that was airflow to the cooling fins above that area once complete.
Not to be outdone, I had a 2.5" 1TB HDD that I could put in a NewerTech Adaptadrive, allowing me to put both the SSD and HDD in the same area (top center). Unfortunately, non-Apple drives (with the exception of some sold by OWC) fail to report HD temperature to the OS, which in turn, causes the iMac's fans to run full throttle. There is software fan control available to answer this, but I found it unreliable, and no substitute to the OS's handling of the fans.
There's also a hardware connector available that goes between the SATA connector and the drive (SSD or HDD), with a thermal sensor. Recently acquired that cable, so I'll be trying again, later.
I also used BootCamp and Windows 7 HPrem. Found Winclone touted as the program to get, so I got it. It worked, almost.
I had both drives in the iMac, buttoned up, and booted holding down the Option key. I had two Macintosh HD's to boot from and two BootCamp partitions to boot from. I booted from each, one at a time, and everything worked. When I pulled the old drive out to let the SSD run on it's own, the Mac drive booted fine, but BootCamp partition did not.
** WinClone purchased, registered, and didn't deliver. After following their directions, when I booted up in BootCamp, blue screen came up saying that it was not a Genuine copy of Windows. Have a nice day. **
I didn't have my Windows disc with me (currently overseas), but didn't think I'd need it because I've download the iso file from Microsoft before, had my activation code (genuine), and as long as I could boot from a USB drive and had an Internet connection, I'd be good.
Not so much. The option to load from an iso file and any reference to a USB boot drive was gone on the iMac (thank you, Yosemite?). Again, looking online, a YouTube video gave instructions on how to get the option back. Still didn't work even after enabling the missing option.
Carbon Copy Cloner worked as advertised.
My next plan, once my wife sends my Windows7 disc, is to put the SSD in the AdaptaDrive, use the thermal cable, and reinstall from Carbon Copy Cloner.
Then, reinstall Windows7 bootcamp the old-fashioned way.
If that goes well, I'll use double-stick foam tape (heat resistant, of course) and put a second SSD in place behind the main SSD. Current concerns are the second drive's SATA cable (red) occupying what's left of the small space alongside the heat pipes, and possibly needing another thermal drive adapter cable for the second SSD. The small bottle-neck where the SATA cables come off the logic board and ride up along the heat pipes is probably the hottest part of the machine. I'm not sure the second SATA cable will last long without a thermal sleeve like the OEM SATA cable (black), and I'm not sure there's enough room to use a sleeve and allow any airflow through that area afterward.
After all that... I guess it comes down to what you really want to do with your iMac. I would guess it'll be good for at least another 3 years. I bought mine at half the cost of a similar new iMac, and I was able to tinker with it. Why a desktop needed to be thinner and lighter I still haven't figured out. Welding was a nifty idea, but not repair/upgrade-friendly.
