Windows – Unexpected free disk space decrease

According to Oracle's zfs documentation,

A snapshot is a read-only copy of a file system or volume. Snapshots can be created almost instantly, and they initially consume no additional disk space within the pool. However, as data within the active dataset changes, the snapshot consumes disk space by continuing to reference the old data, thus preventing the disk space from being freed.

If you want the disk space to be freed, you will have to clear out the snapshots referencing that file. You may also want to read up on copy-on-write, which is how zfs works.

I don't actually have any experience with zfs, but I make use of btrfs. btrfs does snapshots in a similar manner to zfs and the exact same 'issue' appears there. Of course, as I do not want a deleted file in the current working set to be removed from a snapshot, I would certainly expect that the available disk space would remain constant when removing a file. After all, the file isn't actually removed until all references (across all snapshots) are removed.

Few reasons possibly

• write/read cache used on disk
• encryption like BitLocker or TrueCrypt
• Windows may use drive as a dynamic pagefile storing location
• a malicious program running

... or could be none of above.

Did you try to connect to another computer and are the results are same?