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RAID-1 Mirror vs Two Separate Drives for Backup
A RAID-1 mirror enclosure with two drives provides real-time fault tolerance against drive failure; just keep working when one drive fails, replacing the failed rive as soon as possible (and making a backup immediately!).
A mirror is appropriate for when the drive is needed for data storage that is used continually for data storage or always-attached backup.
Examples of appropriate applications for a RAID-1 mirror:
- An external working drive for file storage (music, images, etc);
- An external (or internal) mirror for a Time Machine backup.
- A boot (system/applications) drive.
More separate drives better than fewer mirrors
External backups are backups to be made, then stored safely away from the computer. Compared to one mirrored drive or two separate drives, the two separate drives are better. Or 2 and 4, 3 and 6, etc.
A RAID-1 mirror is not a bad thing for backups, but compared to twice as many single drives, it’s an inferior choice—
- Separate drives each have their own power supply; failure of the power supply affects only one drive.
- Separate drives are smaller and lighter, so it might be practical to carry one in a pocket for portable units, raising the odds of keeping on one’s person a key data drive, thus lowering risk from theft, etc. A mirrored unit is generally too bulky.
- Separate drives can be physically stored in different locations, stolen (or not) independently, damaged (or not) independently, updated with current data (or not) independently, etc.
- When a backup drive is needed for data restoration, a disagreement between a drive and a concrete floor will likely kill all the drives in the enclosure. With a mirror, the mirrored drives are both dead. With two single drives, there is still one more drive elsewhere, besides the one that hit the floor.
- Traveling— with one mirror, loss (theft) or damage can be minimized by storing the two drives separately, e.g., one on your person and one in your hotel room. Portable mini drives are best for travel.
- Separate drives can still use be used as a RAID-1 mirror, but in software. Used in conjunction with the SoftRAID fast mirror rebuild feature, this is is a highly efficient backup strategy.
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Don’t assume a RAID-1 mirror is always better— aside from cost, there are significant drawbacks in redundancy and size/weight.