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.
Yep, that's the problem.
When you bought the drive, it was already formatted for Microsoft Windows. You can use a Microsoft Windows formatted drive to store files, though it can be risky; but you can not boot from a Windows-formatted disk. That's why Silverkeeper was telling you to reformat.