As Dennis Nedry stated: Double check the ID of the internal drive.
It is surprisingly common to find internal hard drives jumpered incorrectly as ID 6. Why? For some models of Mac, in Australia (and possibly other countries?) Apple charged unrealistically high prices for a Mac fitted with a "factory" hard drive. As such, some dealers would offer the option of a floppy disk only machine plus a third party drive. Most third party drives were jumpered as ID 6 out of the box and some dealers employed inexperienced, incompetent or just lazy technical staff to do this sort of repetitive work.
It is surprisingly common to find internal hard drives jumpered incorrectly as ID 6. Why? For some models of Mac, in Australia (and possibly other countries?) Apple charged unrealistically high prices for a Mac fitted with a "factory" hard drive. As such, some dealers would offer the option of a floppy disk only machine plus a third party drive. Most third party drives were jumpered as ID 6 out of the box and some dealers employed inexperienced, incompetent or just lazy technical staff to do this sort of repetitive work.