Attempts to fix this so far have included:
- Replace and increase the machine's RAM.
- Put the USB2 drives on a powered USB hub.
- Replace both drive enclosures.
- Switch from USB2 to Firewire 400 through a powered hub.
- Replace both drive enclosures again.
- Upgrade from MacOS 10.5.8 to 10.6.7.
- Replace the entire Mini itself (macini1,1 → macmini2,1).
None of these have fixed the problem (and #6 has caused another) and at this point, I have replaced every piece of hardware except the power cable and the drives themselves (which fsck fine).
I guess it's time to buy a pair of new drives, unless someone has another idea. (I have put this off until last because copying 1TB of data takes almost two days, each.)
Incidientally, I think I understand the source of the crash/hang: the other day I saw one of the drives get into a situation where "ls" on that drive would hang forever in the kernel and be unkillable with -9, so that's an express train to a full process table and inability to reboot right there. I haven't seen that kind of shit since the bad old NFS days. Apparently my Mac has nostalgia for SunOS 4.1.3.