UPDATE:

Over the past few days, I've found out more information which may or may not help to diagnose the problem.

Three or four times, the hard disk has spun and and stayed spun up. The mac booted quite fine, and, once running, there were no problems. I made various system backup floppies, which I can boot it from, and it did that fine. The disk was like it always was. I did some warm restarts and, as long as the disk kept spinning, it kept working.

But, as soon as I switched the mac off, only five seconds later the disk would not stay spun up. On the times when it doesn't work, there is a quiet "click-click, pause, click-click" noise coming from the drive, about twice a second.

Why does the drive sometimes work perfectly and then not at all? And what is making the feint clicking noise?