I have never had this problem occur for me except twice within the last year and only on Windows 7. I think it has something to do with defraging HDD's and Bitlocker. Both times this occurred was after defraging on a Bitlocker encrypted drive, although I have defragged those drives before without incident. It appears as if the default space (which can be up to 15%) allotted for Shadow Copies and System Restore points suddenly gets maxed out like during the defrag process like Windows starts making tons of these restore points. It may have something to do with Bitlocker Drive encryption. I'll try to do some research on this.