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, modification times,
extended attributes, acls, and resource forks. Also, 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. Finally, rdiff-backup is easy to use and settings
have sensical defaults.