SSD upgrade iMac 21.5

You will need to run some fan speed/ temperature monitoring software since the SSD will not have the Apple firmware in it to report back the drive temperature to the sensors on the logic board. This is the one I recommend: http://www.hddfancontrol.com/ It's not free but is actively updated and supported for new versions of the OS.

If you don't run software to do this, the machine will run okay for approx 30-40 minutes at which time the fans will kick up to high speed and be very loud. This won't hurt anything but will be very annoying and you do run the risk of burning out the fans quicker since they will be running at full speed.

Other than that there shouldn't be any problems running the SSD. Just be very careful when removing the display panel. Because the glass and the LCD panel are glued together you have to be very careful when cutting the adhesive strips that attach it to the rear housing so that you don't crack the glass. As far as I know there is not replacing the glass panel only if you crack it because of the way it's made.

Good Luck