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I'd like to buy 2 USB hard drives and simulate a RAID0 array i.e. plug both drives into my MacBook Pro and have Time Machine back up my data across BOTH drives so that they're mirrors of one another. Can you do this with Time Machine ? I know I could buy a 'proper' RAID back up solution but just buying 2 drives is cheaper...

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RAID != backup. – Chealion Jul 20 '10 at 15:00
up vote 3 down vote accepted

I'm not sure to understand what you need.

RAID-0 consists in grouping several hard drives in such a way that a single volume appears on Mac desktop with a size being the sum of all the sizes of each unit. You can do that with Disk Utility, but your system is less reliable than a single bigger disk (a failure from any of your disks could lead to loss of all data).

Mirroring is obtained with RAID-1, which is possible which Disk Utility again. The size of the RAID volume is the size of the smaller unit.

Time Machine does not mirror hard drives. It keeps an history of changes in the filesystem.


If you want a Time Machine volume that is mirrored on a backup volume, you can use SuperDuper! backup software.

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Basically, i'm wondering if it's possible to plug in 2 USB external drives, and have Time Machine back up files to both of them. I'm not sure (having not used it yet) whether Time Machine can back up files to both external drives, rather than just one. – TanvirK Jul 20 '10 at 13:56
Do you mean that you want a mirrored Time Machine volume ? I have implemented something like that: One external USB drive is my Time Machine volume. A second external USB drive is a clone of the first one and is updated with SuperDuper! – mouviciel Jul 20 '10 at 14:31
Yes that's what i'm looking for, thanks – TanvirK Jul 20 '10 at 14:49

If you have two disks that you can use for backups, it might be a very good idea to use two different backup utilities, too. For example, use disk 1 for Time Machine backups, and then (independently of that) set up Carbon Copy Cloner to take backups to disk 2.

Time Machine is not perfect, it has its bugs, and in some situations it may fail to back up everything properly. Simply mirroring or cloning your Time Machine backups doesn't provide any protection against that kind of data loss; moreover, if your backup volume gets corrupted, cloning it will clone the corruption, too. However, if you use two very different kinds of backup tools (say, TM and CCC), it's much more likely that you have at least one full backup available.

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ok, thanks for the advice – TanvirK Jul 23 '10 at 7:25

Or just use OSX's Disc Utility to create a raid array

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Looking at this link (…) seems to imply you can only create a RAID set from SCSI \ IDE disks. I'm not sure how the OS could treat USB disks in the same way ? – TanvirK Jul 20 '10 at 11:13
Seems to work from – Mark Jul 20 '10 at 12:30
thanks for that link - i'll look into this further – TanvirK Jul 20 '10 at 14:01

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