[Chattered]

But if that external usb drive is mounted at the time (as in the case of the user this thread is about), then all data on that drive will be deleted.

For this reason, the recommened way to use things like rsnapshot is to have your backup directories owned by root and with permissions masked to something like rwxr--r--. If you then want to read your backups easily, you do things like mount it under NFS as read-only.

[Latty]

A (RAID6) fileserver running ZFS with filesystem-level snapshotting. It's really, really good.

[cmurf]

Btrfs snapshots, and then cp --reflink from the snapshot to the current tree whatever files or directories are missing; i.e. no need to rollback to a snapshot.