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We have a Mac Pro tower with an Apple RAID card in it using third party drives. When one drive failed, we replaced it and the RAID 5 set was nearly done rebuilding when the computer was rebooted. It did not come back up.

We are now booting up off of a different internal volume, and have three (third-party) drives of identical spec (including revision and firmware) in the box. One of the drives is a global spare; the other two are recognized as belong to a RAID set but are in "Roaming" mode. The intention is to recreate the three-drive RAID set using the data on the two drives that are good.

When we tell the system to create a RAID 5 using the three drives, it tells us that it'll create a RAID set but everything will be lost. There are no obvious options to rebuild a RAID using the two good drives and incorporating the third drive in Apple's RAID Utility, and we've looked through the options for the raidutil command.

Fortunately, all important data is backed up, and we can rebuild from scratch, but, is there any way to make the RAIDset work again?

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As near as I can tell, the answer is "Tough. There is nothing you can do". We heard that, despite the warning the all data would be erased, if you tried to create a RAID set with the three drives it would rebuild the original RAID set. Deleting it at this point wouldn't hurt anything, and we couldn't see what else to do, so we proceeded. Yep, it deleted all the data.

I thought of deleting the question, but I'll leave it here in case anyone does come up with a solution for future use, or if someone googles it only to find out that there appears to be no solution.

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Ah, the peril of RAID 5. – Tom O'Connor Oct 16 '11 at 20:07

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