I don't really know how this conversation evolved from what you can and can't write to what is on your hard drive, but the most effective way to protect yourself (if you have something on your HD that you don't want seen) is with a hard disk scrubber.

What a HD scrubber does is write over the free space on your disk, usually between 3-200 times with random junk digits. So, after you've deleted everything, wiped your cache, etc., you run the scrubber and it overwrites all the info you just deleted.

Some of the less-thorough ones will leave fragments of files, but not much significant. The really expensive one, like the ones that overwrite the info 200 times, are used by the DOD and the DOJ to make deleted material utterly irretrievable.

Once the deleted bits of information on your drive have been overwritten, they are no longer identifiable by any routine disk scan or sweep. If you have a more effective scrubber, your disk will probably stand up to scans from most law enforcement agencies.