AppleII Gs Problems

[dwilson@mail.nas.com (dwilson)]

 I recently got I working GS. I have a 5.25" disk drive hooked up to it,
and am able to get Apple DOS 3.3 to boot. Today I bought 2 3.5" diskdrives
to go with it. I pluged one into the back, replacing the big floppy drive,
put a GS boot disk in, and turned it on. It starts to read it, then
abruptly stops, and come up with the message " Check Start Up Device ".
Does anybody have any idea whats wrong with it? The 3.5" didn't have a
name on them, but on the bottom said model MO13O. Any help would be
greatly appreciated.