clone won't boot

[camner]

I'm trying (again) an unregistered copy of SD! I just downloaded 1.5.5. Made no changes to the defaults that came up when SD! loaded.

I'm backing up to a partitioned external drive (a USB 2.0 drive).

After cloning was complete, I attempted to select the partition in System Preferences-->Startup Disk. It appears, I can select it, but when I clicked Restart I got a system beep (and the machine did not restart).

I then selected "Restart" from the System Services menu and held down the option key. The partition in question appeared, I selected it and clicked the black "go on" arrow. I got a grey screen, as expected, with an Apple in the middle of it. After quite a wait, the "turning gear" appeared. After about 4 minutes of the gear grinding away, the grey Apple turned into a grey "no" sign (the circle with a slash through it). The gear continued turning. After several more minutes, I then turned off the machine and rebooted, and it rebooted from the internal hard drive.

So, the clone appears to be a startup disk to the system, but the machine won't boot from it.

Am I doing something obviously wrong here? I'm running 10.3.9.

(I should also mention that it took about 90 minutes to clone/backup a drive with about 50 GB on it...is that expected? I'm running a G4 867mHz Dual MDD)