> Some of it could conceivably be "slack space". (A file can only be an 
> integer number of blocks long. The remaining space isn't shown in the 
> "file size", yet cannot be used for any other data because a block can 
> only belong to one file. If you look at file properties, you will see a 
> "file size" and a larger "disk on disk", which includes the slack space.)

I thought of that at first, but, no.  With 4 Kb clusters, the maximum
amount of lost space is rather small (say we got 100 000 files, which
isn't even the case, it would take 400 Mb at worst).

> Do you have that Previous Versions option switched on?

No.  That partition contains only data files, nothing system-related.

It was just clusters erroneously marked as allocated, chkdsk fixed it !

Thanks anyway.

Fabien.

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