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I am getting an iMac 27 inch, and it will have a 1 TB internal hard drive, and I know we can set up the Mac so that this hard drive is mirrored on an external 1 TB hard drive using the built in tools of Mac OS X (Applications -> Utilities -> Disk Utility).

But the OS's system files, the swap file, etc, are all on the internal drive as well. Will those be mirrored as well? If so, won't that slow the system down a lot, since the external drive is usually slower than the internal drive.

Also, even if the OS's files are not mirrored but just the user's own files, will copying or file saving be slower, since instead of copying to the faster internal drive, the OS will report done only after the external drive is done as well?

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I don't think you can create a RAID volume with an external drive. I may be wrong. RAID doesn't really work the way you're describing. Will there be some degree of slowness as the CPU and hardware do their thing? Yes, probably. Will you notice it, and have to wait for all of these tasks to be done before you can continue on with whatever you're doing? Probably not, no. Don't worry about it. –  Harv Oct 12 '11 at 17:42
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A mirrored RAID would operate only as fast as the slowest drive in the setup. In this case, unless you are using thunderbolt I imagine the performance would be awful. Another reason not to do a mirror with an external drive (if Disk Utility would even let you) is it'd be too easy have the mirror fail for a number of reasons.

I would use Time Machine to the external drive rather than mirroring. Remember that RAID is NOT backup! If you mess something up then you will lose data, a mirrored RAID is not going to save you at all. Only Time Machine or a similar backup strategy will help prevent data loss.

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aha, I might have used 2 external hard drives (Western Digital Element 3TB), and then hook them up to the iMac and using Disk Utility to do the mirroring, so that everything happens in the peripheral and so it won't affect the performance of the iMac. I am not so used to Time Machine and it seems quite weird to use it, such as if I create a tmp file of 1GB, and delete it, then it will stay in the Time Machine hard drive. But maybe I will use another hard drive to try out Time Machine and experiment with it –  動靜能量 Oct 12 '11 at 23:21
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