rdiff-backup backs up one directory to another, possibly over a network. The
target directory ends up a copy of the source directory, but extra reverse
diffs are stored in a special subdirectory of that target directory, so you
can still recover files lost some time ago. The idea is to combine the best
features of a mirror and an incremental backup. rdiff-backup also preserves
subdirectories, hard links, dev files, permissions, uid/gid ownership (if it
is running as root), and modification times. Finally, rdiff-backup can operate
in a bandwidth efficient manner over a pipe, like rsync. Thus you can use
rdiff-backup and ssh to securely back a hard drive up to a remote location,
and only the differences will be transmitted.