Losing Free Space

Normally when I run Degraggler (V2.18.945) I gain some free space on my 1TB hard drive (C:)

Yesterday I lost approx. 50GB of free space.

I am running WIndows 7 Pro, 64Bit

By the way, I have run Windows Chkdsk on a restart and it did not report any disk errors or bad sectors.

I have not downloaded and large files recently or installed / updated any software.

Please confirm you have read the stickied post regarding the most common reason for this

http://forum.piriform.com/index.php?showtopic=35459

Stop VSS when defragmenting NTFS volume was unchecked.

I checked it and reran Defraggler.

I have regained the lost 50GB.

Thank you for your assistance.