ANY backup solution can go down with a big enough disaster. An on-site backup is A backup, but it shouldn't be the ONLY backup. Typically you want a hierarchy

ANY backup solution can go down with a big enough disaster. An on-site backup is A backup, but it shouldn't be the ONLY backup. Typically you want a hierarchy of backups, one of which is on-site and on-line but a different machine, another of which is off-site but on-line, and possibly one or more offline backups. But on-line backups are fine, IF (and only if) they're copy-on-write/append-only. Of course if the data can be deleted or overwritten too soon (even tape drive backup systems have some period over which they start re-using tapes) it's worthless. But that's an entirely separate concern from whether it's always-on or internet-connected or on the same national power grid segment as the original.