Update: OnTrack couldn't do anything. Stellar Phoenix did the job. I lost about a week's worth of updates, but other than having to ask my students to bring in their midterms so I could re-record those grades, nothing important. My photos lost their metadata but I still have the images, which is what matters most.
The only problem is e-mail. Apparently Apple changed its Mail program to use a new .emlx file format but neglected to update it so it would import .emlx files. This is a known problem. Several people have posted solutions online, but so far none of the ones I've tried have worked. (The emlx to mbox converter shareware program just hangs when I try it, on both G4 and Intel laptops running OS 10.4.11.) Any ideas in that department would be gratefully appreciated.