Other things fail
For several weeks, I've been working to fix a lame hard drive on one of Eastgate's G5 towers. I started by making lots of backups, and then added a new internal drive that would (I thought) replace the failing internal drive.
The new drive was also flaky. Lots of time with Disk Utility and Disk Warrior, lots of fiddling, lots of warning dialogs. Bad! But I'd bought on price, and I figured this was the penalty. So we bought another new hard disk.
Same results: flaky disk, bad news all round. I started worrying about bad cables, or a bad motherboard. I started budgeting for a replacement CPU.
So I bought an external firewire to make another solid backup. Returning from Australia, I tried running from the external drive. It was error-prone, too. This was getting ridiculous.
The answer: bad RAM. One of the DRAM modules was erratic! The occasional memory error generated occasional disk problems, and away we went. Yank out the bad memory, and things seem to be OK once more.
Yet another reason to back up early and often.