I am in the middle of a "drive wiper > wipe free space" wipe, and the "properties" of my drive C reveal that I have very little "free space" (15.7 gb out of 889 GB) compared to before (over 400 GB free space) I had started the wipe (multiple passes). Is this normal, and should I be alarmed? I was under the impression that I would not lose any free space, that the CCleaner just checks the free space for data that hasn't been overwritten. Thanks!
It's normal. CC 'cleans' free space by creating new files that fill the disk, then deleting them. Your free space will return to what it was when you started if you let CC run to completion.