On the machine where I had the problem I had to run memtest86 more than a day to finally catch it. Then after replacing the RAM and fsck'ing the volume, I still had mysterious problems about once a month until I realized that the disks are accessed alternately and the fsck pass didn't catch everything. I forget the commands to compare and fix the mirroring, but they worked - and I think the centos 5.4 update does that periodically as a cron job now. The other worry is that when one drive dies, you might have unreadable spots in normally unused areas of the mirror since this will keep a rebuild from working - but the cron job should detect those too if you notice the results.
I am going to take a good look at the cron jobs on the moodle box then. Need to check whether the ubuntu box does the same. Man, if only I had a Centos cd when the previous gateway died...