Windows 7: Windows Media Player 12 Problem: Music Library Looses MP3s

32bit Windows 7 is on C: and all my music files are in F:\Music (F: is another non-removeable disk drive).

Using Organize (in WMP) I've specified F:\Music is a music library & consequently all files in F:\Music appear in WMP as expected. However, if I close WMP and then reopen it, the library F:\Music is still connected (according to Organize) but the files in F:\Music are no longer visible inside WMP.

I can fix it by using Organize to remove the library F:\Music then click OK then go back into Organize and add library F:\Music again.

Any suggestions how to fix the problem permanently?

Thanks.