I just tried out Windows Backup (7/64bit) and it behaved VERY strangely. I was backing up to a large array. The actual backup data was about 90gb total. When the backup completed the box said there was about 200gb free on the target drive--total insanity.
I already had Total Commander open, I switched over to the backup drive and started looking for some bogus file--I find nothing and then I note the free space is about 1tb. Nothing has been written or deleted! I ask for the space used by a directory, get a reasonable answer and I note the free space changed again.
At this point I hold down the refresh button--the free space on the drive is going up by many gb/sec.
Outside forces intervened, by the time I got back to my computer the free space was what it should have been and no longer changing.
What borrowed over 5tb and then gave it back over time????