[bigiain]

Another commenter mentions the setup involves a Pi with an SSD and two USB sticks. It might be configured so you're required to switch usb sticks for consecutive backups, so you'd always have at least your second most recent backup completely offline - or even have the system ensure there's only a backup usb stick instead while the backup is running (and either unmount it of nag you to take it out when the backup is done).

[nemiah]

I'll actually be sending out emails if there are more than 14 consecutive backups on the usb stick to remind the customer to switch them ;)