Can you please let us know if you can recover any data from the disk after wiping it?
OK, you seem to have enlisted me as an unofficial tester!
I used the WinHex editor to establish what is actually on the disk at various stages and CaptureWizPro to record videos of CCleaner in action so I can catch exactly what it is reporting.
Firstly the bug does seem to depend on the exact partition size - in trying to replicate my earlier results I tried sizes of 3031MB, 6062MB and 5155MB (all as reported by CCleaner as the total size of the free space when newly formatted). All of these seemed to be OK i.e. the wipe progressed linearly to 100%. However when I reset the size to exactly 5151MB as before, the problem recurred. On the 35-pass wipe the count reached 4127MB (80%) and then jumped directly to 5151MB (100%) fleetingly before returning to the start screen. On the 3-pass wipe the count reached 4445MB (86%) before jumping directly to 100%.
To see what was actually happening on the disk surface I did the following:
1. Reformatted the drive.
2. Created a TrueCrypt container file 5150MB in size in the root directory.
3. Deleted this file and emptied the trash.
4. Used WinHex to confirm that the entire free space was filled with pseudo-random data.
5. Executed a 3-pass free space wipe in CCleaner (v3.02.1343(64-bit)).
6. Examined the result in WinHex - most of the free space had been zeroed, but there were very significant zones throughout still containing pseudo-random data.
7. Executed a 1-pass free space wipe in CCleaner.
8. Examined the result in WinHex - now *almost* all the free space had been zeroed, however a 578 cluster block right at the logical end of the disk had still been missed.
I have not tested the 32-bit version, but intuition is telling me this is probably an integer truncation problem introduced in the conversion from 32-bit to 64-bit and if I had access to the source this is the first thing I would be looking for.
I have attached a video of the tail end of a 7-pass NSA wipe so you can see the problem for yourself.CCleanerBug.wmv
Happy hunting,
John.