Data Recovery Case File · NAS & Network Storage · Each Disk Is Complete
One Disk Failed and Both Copies Exist
His enquiry contains a fact that should make this straightforward and a symptom that suggests it is not. A two-bay network unit with two 2TB disks: "one of the drives has failed. I can't access the system now, even though I remember the system has a mirrored setting." He is right to be puzzled. A mirror is the one arrangement where a single failure should cost you nothing — because each disk holds a complete, independent copy of everything. So a mirror that is entirely inaccessible after one failure is telling you the problem is somewhere other than the data.
| Media | Two-bay network storage unit in mirrored configuration with two 2TB members — one member failed; volume inaccessible |
| Reported situation | Two-bay unit configured as a mirror · one member reported failed · system no longer accessible · both members retained · personal content held |
| Fault class | Mirrored set with one member failed — surviving member holding a complete copy; inaccessibility indicating appliance or second-member fault |
| Equipment used | Both members removed and labelled by bay · surviving member imaged write-blocked and read as a standalone copy (Atola TaskForce 2) · appliance electronics assessed separately · failed member imaged individually where required · no rebuild attempted |
The decode: what a mirror is, and what the symptom implies
Why a mirror is different from everything else in this archive: striped and parity arrangements break files into fragments distributed across every member, so no single disk holds anything usable on its own. A mirror does the opposite — it writes the same data to both disks simultaneously, so each one is a complete, self-contained copy with a full filesystem on it. That is the entire point: either disk alone contains everything.
What that means for him: the surviving disk should hold a complete copy of his data, readable independently of the unit, the failed disk and the array configuration entirely. It does not need reconstructing, because there is nothing to reconstruct — a mirror member is not a fragment.
So why can he not access anything? Three possibilities, and none of them is "the data is gone". The appliance itself may have failed — its power supply or board — which is common on units of that age and would make both disks unreachable regardless of their condition. The second disk may also be failing, which is far more likely than it sounds because mirrored disks are bought together, run together and receive identical write loads, so they age at the same rate and fail at similar times. Or the unit may have refused to bring the volume up degraded, which some appliances do rather than serving from a single member.
Why establishing which comes first: if the appliance has failed, the surviving disk is read directly and the job is straightforward. If the second disk is also failing, it needs proper handling — and the failed disk becomes relevant again, because between two partly readable copies of the same data, the readable portions can be combined. Both disks should be kept for exactly that reason.
The thing that must not happen: no rebuild, and no fitting of a replacement disk into the unit. A rebuild reads the surviving member end to end, at maximum load, for hours — on the disk that currently holds the only copy, and which is the same age as the one that just failed. That is the worst possible moment to give it the heaviest work of its life. And no member should be connected to a computer that offers to initialise it.
The point worth making about mirrors generally: a mirror protects against one disk failing. It does not protect against deletion, corruption, the appliance failing, or both disks reaching the end of their lives together — which is why a mirror is not a backup, however reassuring two copies feel.
On the bench
Both members were removed and labelled by bay. The surviving member was imaged write-blocked and read as a standalone copy on the Atola TaskForce 2 — a mirror member carries a complete filesystem rather than a fragment, so it needs no reconstruction and does not depend on the appliance, the configuration or the failed disk. Appliance electronics were assessed separately, since a failed unit makes both disks unreachable regardless of their condition. The failed member was imaged individually where required, and no rebuild was attempted.
The outcome
The surviving member read as a complete standalone copy, the appliance assessed separately and both disks retained. Free assessment, one fixed written figure including VAT; where a drive has to be opened, 50% of parts and labour is payable upfront with the balance only on success — otherwise no recovery, no fee. The decode, for anyone with a mirrored unit that has lost a disk: each member of a mirror is a complete independent copy, not a fragment — so the surviving disk should hold everything and can be read without the unit, the configuration or the failed disk. Being unable to access anything therefore points at the appliance, or at the second disk also failing, which is common because mirrored disks age identically. Keep both, and never rebuild.
Mirrored unit inaccessible after one disk failed
Keep both disks and don't let the unit rebuild. A mirror isn't like other arrangements: it writes the same data to both disks at once, so each one is a complete self-contained copy with a full filesystem on it rather than a fragment. Your surviving disk should hold everything, readable without the unit, the configuration or the failed disk. So being unable to access anything points elsewhere — most likely the appliance itself has failed, or the second disk is failing too, which is commoner than it sounds because mirrored disks are bought together and take identical write loads, so they wear at the same rate. That's also why a rebuild is the wrong move: it gives the disk holding your only copy the heaviest read load of its life, at the worst moment.
Keep both and don't rebuild — call Newcastle Data Recovery on 0191 406 1051; surviving member read as a complete standalone copy, appliance assessed separately, no rebuild attempted.
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Our case files are drawn from genuine enquiries received by our laboratory over the past ten years, anonymised to protect client confidentiality. Each one describes the diagnostic and recovery procedure our engineers apply to that fault, using the equipment listed.