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Mac OS RAID 5 recovery

Hardware and RAID configuration

This was a 16x500GB RAID 5 on Mac OS.

Problem:

The customer needed to recover data from the RAID that might have been rebuilt. The customer was able to use RAID recovery programs to reconstruct the RAID and scan the HFS+ volume. However, only small files were recovered. All large files were corrupted.

Diagnosis:

Our extensive RAID analysis suggested this was originally a RAID 5 with 14 drives. Some other interesting facts were found during the analysis:

  • One drive was stale.
  • One drive was completely empty. It could be a spare.
  • One drive contained valid data at the beginning but corrupted data towards the end.
  • One drive had two parts with each part containing the mirrored data from another drive.
To reconstruct data from a 14-drive RAID 5, at least 13 good drives were required. We had only 12 good drives with current data.

Solution:

We used special techniques to carve out and combine usable data from the bad drives. The result was the equivalence of a 13th drive.

Result:

The folder structure was 100% recovered. Less than 1% of the recovered files were corrupt. They were the most recently created or modified files.

Lessons learned from this case:

  • A RAID can be recovered successfully even after several attempts of unsuccessful rebuild.
  • The sooner the customer contacts an expert for help, the greater the chance of a successful recovery.
  • Large RAID systems with many drives usually require special knowledge and techniques for a successful recovery.


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