Thanks SB - after my post I started probing the Apple help files (I'd been relying on LaCie's which I didn't find quite so helpful to the technically naive) and a phrase with the words "stick" and "wrong end" came to mind....
No, I hadn't formatted it - just started loading stuff on. The Apple files helped me sort out the difference between volume and drive and the light came on in my brain. Previously I've just backed up by copying important stuff to an external drive. With the G5 getting a bit long in the tooth, decided to (finally!) do the job properly. Fortunately I had space to move the contents back out, so it is now empty again.
If I've now got it right, what I should do is format the Quadra, creating a 250GB bootable partition for the backup. The rest of the drive is then available for general use and I can use Silverkeeper properly....
Hopefully you can help me with another query; I found a site with transfer rates, and it appeared it would take about 1hr 15min to transfer around 200GB over Firewire 400 - does that sound about right? I would have expected it to take a lot longer. But maths was never my strong point
In case this helps: The Mac has never been set up for multiple users - there's only the two of us and mostly it's me that uses it.
TIA