It's nothing too special. I use a separate Aperture library per year, each stored on an external hard disk. It's a Western Digital "My Book Edition II" with 2 x disks in RAID1 configuration for redundancy. This is configured to backup to Amazon Glacier using Arq, which is clever enough to only backup when the drive is connected and only uploads the new or changed files. Works quite effectively and once configured I don't really need to think about it.