I bought, what I thought, would be a clever little dongle: a small USB device to accept SDCards and such. Must have been cheap, rather than inexpensive. I used it to transfer some files. I made the mistake of attempting to use it to delete files as well. Arrgh. It corrupted the file system. Arrgh. I tried a bunch of utilities. Things like GetBackData and such. They could find nary a thing.
A Q&A at StackFault gave a pointer to .... PhotoRec, a photo recovery tool.
The download is actually two applications: one is a file recovery tool, and the other is a photo recovery tool. The file recovery tool is what I tried first. It failed miserably. I thought I lost a whole evening's shooting for a band.
Later the day, in following links, I realized I hadn't tried the PhotoRec tool. I should have tried that first. It is totally amazing. It works by skipping the file system organization, and goes directly to scanning disk sector blocks. Photo files have signatures embedded in the file. The PhotoRec tool scans looking for those signatures, rebuilds the file, copies, and then places the file on a different drive.
It is still running while I write this, and still recovering files. I think I had around 700 files to review. Around 200 have been recovered so far.
Totally amazing. I think I need to do a donation for that one.
Lesson learned on this one: do a total read-only copy first, before going around and messing with files.