In addition to my "online" storage pools, I keep a three-way ZFS mirror on encrypted hard drives of everything that is irreplacable. If you have more than fits on one disk, maybe use a 1+0 or 1+5, even, setup. This story does depend upon you being able to connect two complete copies of your backup to one machine at a time, so again, if it's really a lot of data, external RAID enclosures may be in your future. In any case, one of the full copies lives off-site and every month I grab one of the two at home, swap it with the off-site one, allow it to come up to date with the third, and then push that month's backups to both. Rinse, repeat, like clockwork.