Originally Posted by
brewbuck
Open a file. Start writing random data to the file. Keep going until the OS gives you an error because you ran out of disk space. You have now overwritten all free space on the drive, with random data. Now delete the file you just created.
EDIT: If you're looking for security from the cops -- good luck. Not only will they find the little bits you didn't catch, but there is technology which can read old bit states to a reasonable degree of precision -- i.e., they can read data you've overwritten with something else. That's why industrial strength data erasure tools make multiple passes with different bit sequences. And if you want real security, smash the drive and then burn it.
EDIT EDIT: For instance, did you clear your browser cache? If not, the free block eraser won't erase those sectors, and you might get "busted." I really question the legitimate need for pure data erasure. The big boys just destroy the drives when they need data to go away.