Creating a Backup of a Mac hard drive that won't boot?
If the volume mounts, you may be able to use rsync to back up as many files as you can get. You'll have to make sure you have enough space for them on your iMac G4.

The command you'll run will look like this: (make a new folder on your desktop to put all this in)

rsync -avxr /Volumes/Macintosh\ HD/* ~/Desktop/newfolder

This will attempt to move everything inside Macintosh HD to that new folder. It will skip over the things it can't transfer and keep going. With a bad volume like yours, this may be your best bet.

Good luck!