Quote:
...so the fact that some of the files are not there shouldn't cause a rebuild.


But it does -- the player checks that the count of FIDs on the disk matches the count in the database file. They don't match, so it'll rebuild.

As others have noted: the rebuild is in memory only. What's happening is that the player rebuilds the database in memory, and then attempts to write it to disk. Since the disk is read-only, it isn't written, and needs to be rebuilt next time. If (as the linked FAQ says), you have the disk mounted read-write when the player rebuilds the database, it will be written to disk, and won't need rebuilding next time.
_________________________
-- roger