I've had minor issues swapping cd drives over due to the mounting in a couple of units (HP or IBM IIRC).
The drives came on a slide tray that clipped in rather than have standard mounts or holes. Did not take long to add holes and use a converter tray that fits into a normal 5 1/4 inch drive bays and mounts 3.5" or 2.5" drives.
I did see one Wyse sff machine that had a laptop cd in it - no way could I fit anything in that :-)