IF you are running Vista, this can "hide" files unexpectedly if they are under:
C:\Program Files\ [anything following]
C:\Documents and Settings\ [anything following]
We had to move our "working files" to C:\MTU to get Vista to leave them alone.
BTW: I don't think your WMA file problem is related to Hoster if you are not using 3.318 or 3.319. These could exercise a Windows problem that would trigger Windows to eat files instantly... 65,000 disappeared in a fraction of a second in one case.