Thought I would post this -- did not find any mention of the problem elsewhere.
I have Sansa Clip+ 8GB with Rockbox 3.13. I loaded with files via Media Monkey. It played from the Files directory with no problem. But when I selected Database and chose Initialize, the progress count was extremely slow, like 1-2 files per minute. Looked at every setting, restarted many times, saw nothing. Also noticed that I could not save the config file.
Finally when I plugged into laptop in USB node, happened to check space -- zero bytes! Deleted a large artist directory ~100 Mb, and now Database Initialize runs like a demon, and everything works fine.