[dnanian]

That's unusual. We'd only expect an error like that if the drive itself didn't support booting, or if you were trying to boot from an image.

If you could send your entire SuperDuper!.log to support, I'll take a look and see what I can figure out. I'd also love a System Profiler report, if you could -- open System Profiler and choose File | Save, then save to the default (XML) format. Under Jaguar, that might not be possible, but whatever you can do would help. Make sure the external drive is attached.

You can tell which drive is the boot drive by looking at its root: the boot drive has icons for "Applications" and other folders, whereas other drives don't. You can also tell in System Preferences | Startup Disk -- the default boot drive is selected when it first comes up (although that isn't always the 'current' boot drive if you used Option).