Why should my mirror volume use 3 disks?

We recommend that mirror volumes use 3 disks. Here’s why each disk matters.

Disks 1 and 2: Your always-on mirror pair

The first two disks form a traditional mirror — if one disk fails while you are working, the second immediately takes over, and you can continue without interruption.

These two disks should be connected to your Mac at all times. They can be internal disks in a Mac Pro, drives attached to a Mac Mini being used as a server, drives in a ThunderBay or Mercury Elite Pro Quad enclosure, or any pair of reliable external disks. SoftRAID can mirror disks connected over any bus — USB, Thunderbolt, or SATA — as long as the disks are reliable.

Disk 3: Your off-site disaster recovery disk

We recommend that the third disk be an external disk that is normally stored in a separate building. Periodically bring it back to your Mac, reconnect it, and let SoftRAID rebuild it so it gets updated with your most recent files. Then return it off-site.

This third disk is your disaster recovery safety net. If your Mac is stolen, or the building it’s in experiences a fire or flood, you still have a current copy of your data stored safely elsewhere.

At SoftRAID, we reconnect our third mirror disk every two weeks. That way, the most we can ever lose is two weeks of work.

A note on RAID and backups

RAID — including mirror volumes — is not a substitute for backups. RAID protects you from a disk failure, but it does not protect against accidental deletion, file corruption, malware, or a site-wide disaster affecting all your on-site disks simultaneously. A third off-site mirror disk goes a long way toward real disaster recovery, but a complete backup strategy should also include regular off-site backups of your most important data.