We use a 2TB usb mounted Western Digital drive (I have 4), when the first 1 fills up, I unmount it, tag it and send it over to our offsite facility. I then mount a fresh one, making sure the permissions are set to zimbra:zimbra and continue on.
Our fulls are on a Saturday, so I swap out on a Friday afternoon. When I need to restore, I grab the correct drive and re-mount it.
Works very well!
Ben Franklin quote:
"Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety."