RAID 1, also known as a mirror or mirroring, is one of the many RAID levels supported by OS X and Disk Utility. RAID 1 lets you assign two or more disks as a mirrored set. Once you create the mirrored set, your Mac will see it as a single disk drive. But when your Mac writes data to the mirrored set, it will duplicate the data across all members of the set. This ensures that your data is protected against loss if any hard drive in the RAID 1 set fails. In fact, as long as any single member of the set remains functional, your Mac will continue to operate normally, with complete access to your data.
You can remove a defective hard drive from a RAID 1 set and replace it with a new or repaired hard drive. The RAID 1 set will then rebuild itself, copying data from the existing set to the new member. You can continue to use your Mac during the rebuild process, because it takes place in the background.
RAID 1 Is Not a Backup
Although commonly used as part of a backup strategy, RAID 1 by itself is not an effective substitute for backing up your data. Here’s why.
Any data written to a RAID 1 set is immediately copied to all members of the set; the same is true when you erase a file. As soon as you erase a file, that file is removed from all members of the RAID 1 set. As a result, RAID 1 does not allow you to recover older versions of data, such as the version of a file you edited last week.
Why Use a RAID 1 Mirror
Using a RAID 1 mirror as part of your backup strategy ensures maximum uptime and reliability. You can use RAID 1 for your startup drive, a data drive, or even your backup drive. In fact, combining a RAID 1 mirrored set and Apple’s Time Machine is an optimum backup method.
Let’s get started creating a RAID 1 mirror set.