The zeroer utility can be used to wipe empty space on a disk. In contrary to dd,
zeroer does not wipe existing files on a partition. It overwrites the
unallocated disk space around existing files, which means that deleted files
cannot be restored after processing a certain partition with zeroer.

The utility's principle consists in writing huge zero-padded memory blocks to a
file. To a certain extent this works similar to dd, however zeroer dynamically
reduces the blockwriter's buffer size when the filesystem is going to be full.

WWW: http://critical.ch/zeroer/