techknight: Yeah, they're drives taken from a PC. However, I do have another drive that was also taken from a PC, an IBM 2GB, and the machine has no trouble booting off that, so I don't think it'd be an issue with Apple ROMs.

As for the solder bridge, you guys aren't going to believe this, but I just pulled the interface board out of the expansion bay module...at some point someone has already applied the solder bridge to the board, but it seems he/she has bridged the wrong pins together - according to macdan you're supposed to short pins 45 and 47 together - it seems that they've done 43 and 45...hmm, we might be onto something here...