I had an external case that helds two 5.25 floppy drives that I used with my Sinclair ZX81. I converetd it to hold a 3.5" 40 MB SCSI (Connor as I recall) drive and a Chinon CD-ROM drive that I used with my upgraded 128K SCSI Mac. I could never get it to see or install/boot from the hard drive. After a week of trying, Conner tech support said the problem was the power supply.
I ended up buying a new SCSI cab w/ power supply. With the new power supply it worked like a champ! Ever since then I have been leary of using floppy drive power supplies for hard drives.