Many thanks for your response Dave.
The only thing that could have interfered (but should not have) with my backup was using my 'iMac' partition of the external firewire drive to backup my brother's hard drive while I installed Tiger on his old iMac (it only has a CD-ROM slot, not DVD, so I used my G5 iMac to install Tiger on his system). I then re-loaded his files onto his iMac and used SuperDuper to backup my iMac once more to its partition on the external firewire drive.
None of this should have affected the Powerbook partition of the external firewire drive, though, should it?
Are there any settings I should check to be sure?
Tricia