While the file folders on the Clip+ itself were apparently not affected, some of the ones I loaded onto the micro SDHC card showed strings of random numbers in the file names and .ofn files were generated. I tried the solution in this thread, but changing the settings created a different problem–when connected to the computer, I could no longer see ANY of the files on the Clip+ itself, which showed up as one device. The card showed up as a completely separate device, and I could see the files loaded there. (Previously, the Clip+ had shown up as a device with internal memory and external memory.) I also had the “safely remove” icon now, which I didn’t have before.
The files still existed on the Clip+ even though I could no longer see them. Obviously, unacceptable. So I went back into settings and changed the USB setting back to the factory settings (autodetect). Now in some of the folders on the card, I suddenly had .sid files–I’d have song1.mp3, song1.mp3.sid; song2.mp3, song2.mp3.sid, etc., for all the files in a folder. These .sid files were all 0 bytes.
So … I’ve cleaned up all the folders on the card, getting rid of the numbers, and I’ve deleted all the .sid files. I disconnected the Clip+, waited for the refresh to complete, and then reconnected to the computer. And the random numbers and the .ofn files are back on the card.