Success!

It probably would have made more sense to use copy commands in update.txt but I had already copied the files to the mounted 83.bin (after making a backup, of course). It took about an hour to copy 2GB to the SD card and then to the internal flash. But now everything is back to normal. Thank you very much for the help.

The strange thing is that even though webupdater.exe copied the 4.0 GUPDATE.GCD to .System, the Nuvi is still on 2.30. I suspect my original problem was related to a badly-timed reset that corrupted the file system rather than an incomplete upgrade. I'm not sure if I should try copying fw_all.bin to region 14 or leave well enough alone .