TFAT based FS,media is FULL

Hi *,

The Flash filesystem runs out of free sectors when suddenly powering
it off.
The debug output is like this:
0x83fe3b48: FLASHDRV.DLL:SM_GetNextFreeSector() - Unable to retreive
free physical sector... ALL OUT!!!
0x83fe3b48: FLASHDRV.DLL:CP_CompactBlock() - Unable to get next free
physical sector address for writing! The media is FULL

The target is a MIPSII, WinCE 5.0, QFE updated.
The FS is NOR flash based running TFAT, using the MS FASL driver.

From testing I can see that it gets worse and worse, after each reset
during application run.
The FS simply looses free sectors, even though I'm using the same
files over and over again.
The FS doesn't get corrupt, just full. All file functions return
(The test application does WriteFile to 20 files with a size of 10KB,
allmost as fast as possible).

Any ideas on this issue ?.