Case file · NAS & RAID · EDR-2025-2642
RAID 5 With Two Members Down.
From a fabrication firm outside Yeovil, a six-bay shelf that failed in stages: one bay had been amber for a month
, then a second dropped out during Friday's backup. The man who minds their machines put a cold spare in and set it rebuilding
; by morning it had frozen part way
. Half the damage was done.
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In plain English.
The arithmetic of RAID 5 is unforgiving: parity absorbs exactly one loss. A second failure leaves the controller with equations it cannot solve — and telling it to rebuild anyway makes it write fresh parity across stripes that recovery needs preserved exactly as the crash left them. The second hazard was the shelf itself: enterprise disks often carry unusual sector formats and vendor metadata that desktop equipment mangles or refuses. And the quiet lesson underneath: that amber light had been announcing this fortnight for a month, with parity covering for the limping disk the whole time.
The tools this job took.
See how your case runs →| System | The part it played | Why we rate it |
|---|---|---|
| PC-3000 SAS/SCSI | Diagnosed and read the six enterprise members across their own native interface | Because SAS and SCSI hardware from server rooms is beyond ordinary desktop ports |
| Atola TaskForce 2 | Cloned members in parallel rather than one at a time, saving days | Clones multiple array members at once, shrinking a week's imaging into days |
| UFS Explorer RAID Recovery | Recovered the geometry and mounted the rebuilt volume from the images | Reads NAS volume managers as the layered systems they are, not one flat array |
On the bench.
Six disks, six images — the living included
Nothing was trusted to the shelf again. All six members came out, were labelled by bay, and went to the imagers — survivors first, casualties after assessment. Getting the SAS interface and the enclosure's sector formatting right at this stage was the foundation; a mistake here poisons every later step.
Coax the two casualties into giving up their surfaces
One casualty had a weak head, the other a growing crop of bad sectors; neither was anything like empty. Careful handling brought large majorities of both surfaces into the images — and with both on file, the rebuild could take whichever copy of a disputed block read cleanest rather than trusting parity alone.
Let the on-disk metadata dictate the rebuild
Member order, stripe width, rotation and delay came straight out of the array's own metadata — no defaults, no guessing. Assembly happened virtually, across the six images, and the filesystem was walked out of that virtual volume with the originals untouched.
How it ended.
The volume mounted, verified, and went home on the firm's cold spares — demoted to plain delivery disks, the one role that suited them. Honesty requires the asterisk: where the overnight rebuild had re-written stripes, part of a single project folder was past recovering.
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