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RAID 1 Mirroring
When a drive fails, operation continues seamlessly with the other drive: no data is lost. The failed drive can be replaced, and the mirror can rebuild, automatically (this is one oddball way to backup: swap one of the drives in the mirror with a blank one).
A RAID mirror is a single volume that looks and behaves exactly like a single “drive” (volume): one icon on the desktop. That is the beauty of a mirror: there is absolutely no difference in usage as compared to a single drive/volume, yet failure of one drive maintains all data and functionality.
Typically a RAID-1 mirror consists of the entire capacity of all the drives in the mirror set but it is possible to mirror partitions across drives.
* The term “mirror” is historical and apparently chosen by someone who never looked into one (hint: try to read printed matter in a mirror). A better term would have been “RAID Replica”.
2 or more mirrors
If there are N drives (partitions) in the mirror, then N-1 of them can fail with no loss of data. The most common setup is with N = 2 drives.
A RAID-1 mirror can be created in software with two or more independent drives, or via hardware such as a dual-drive unit like the OWC Mercury Elite Pro Dual.
Mirroring generally runs at the speed of a single drive or a bit slower for writes (all drives can be written simultaneously, but there is some overhead). A mirror can offer higher read speed, but not necessarily.
Backup is still required
A consistent backup strategy is critical, even with mirroring: Acts of Dog can occur (fire, hurricane, lighting bolt, etc).
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Creating a software RAID-1 mirror
See the RAID-1 how-to page.