Something is messing up the drive detection. There is UBoot detection and there is OS4 detection. Easiest may be to simply reset the NVRAM ro default from UBoot menu and set boot order again.
It's hard to know what the machien has been through, what changes have been made and what controller it was booting off.
It tends to be some OS4 NVRAM variable that sets a unit to one drive type where another sits. CD/DVD such as. If OS4 find the type is wrong it just rejects the drive leaving you clueless. :-)
