One drive has failed in my RAID 5 array, which contains roughly 40 TB of data. I'm planning to build a new array with at least RAID 6, but I'm unsure whether I should first replace the failed drive and rebuild the current array, or move the data directly to the new array while the old one is degraded. The source array is still readable, although one drive is struggling, and the migration will take a long time. I also tried installing a replacement drive, but the enclosure showed no activity and did not automatically start a rebuild, so the drive bay or RAID configuration might need attention.
3 Answers
Replace the failed drive and rebuild the current array before migrating. A degraded RAID 5 has no remaining redundancy, so another drive failure could make the entire array unavailable. Once it is healthy again, copy the data to the new RAID 6 array. Make sure your backups are current before stressing the disks with a rebuild.
If inserting the replacement did nothing, check the RAID management interface rather than assuming the disk is bad. Some controllers require you to mark the disk as a replacement, hot spare, or online member before rebuilding begins. Also test the enclosure slot, backplane, cabling, and replacement drive, since the bay itself may have failed.
There is a real risk either way. Rebuilding a large RAID 5 puts heavy load on every remaining disk, and an unrecoverable read error or another failing drive can destroy the array. Copying 40 TB from a degraded array also takes time and keeps the data exposed. If you have a verified backup, use whichever process is easier; without one, replacing the drive and rebuilding is generally the safer first step.
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