I did the same thing once about 5 years back. Bought about a dozen 2.5" drives from a garage sale. I had all manner of trouble getting them working until I found that I had to set the drive Master/slave jumpers just like I did on a 3.5" drive. After that I never had any trouble with 2'5" drives in an Amiga.
Each drive was different. Some wanted a jumper, some where find without. I just experimented with each drive until I found the right jumper position for that drive.
