I like to overwrite the disk free space with zeroes so when I make a compressed image of the disk for backup purposes it will compress the unused portions of the disk very effectively. Does anyone know if the CCleaner single pass method uses zeros or random data or ?
I'm wondering what software you use for "Imaging" your System Drive?
There are very good free Disk Imaging applications available which do not include free space in the Image, therefore keeping the size down.
One used by a number of members on here is Macrium Reflect. (Free Version)
As to your question, I believe CCleaner overwrites with 0's, but I'm not 100% on that, as I've never seen it actually written down in any of the guides.