So, I bought a case for my old hard drive, plugged that in and booted the computer using the old hard drive using USB 3.0 to rule out the SATA cable.

Neither Mountain Lion nor Mojave have affected the speed. Thinking it might be that Catalina doesn’t run on my old iMac, I reformatted the drive, and installed what came with my iMac, Mountain Lion, then updated to Mojave (which I ran on the old Hard drive). Hooked everything back up, installed Catalina and my computer all of a sudden was SLOW and super loud fan noise. As suggested, I installed the thermal adapter on the SSD as well.

Installation was not nearly as difficult as I expected, and had no problem installing all the items based on the guides provided. So I ordered a new LiteOn powersupply through ifixit and decided that if I am opening the iMac, I might as well change my 3TB HDD drive, with a 2TB SSD to speed up my old Mac. I had issues with my late 2012 iMac randomly powering off, and the culprit of this seems to be a broken power supply. I’ve probably read just about every post on the internet regarding my issue and cannot find an answer so I am hoping the ifixit community can help.

Enable trim mac os x 10.8.5