Data Recovery Case File · NAS & Network Storage · Members Are Not Disks

Two Array Members, Repurposed by Mistake

His enquiry described a reasonable act of tidying that had an unreasonable consequence. A set of drives that had been an array, retired when he bought new ones: "before I did that I created a separate FAT32 partition on two of the drives, both 2TB each. Now for some reason when I try to boot into them or read them from my laptop I" cannot. Two things are true at once here. Partitioning those drives did real damage to the set, because array members are not independent disks. But a partition table is a tiny structure at the very start of a disk — so what was written is small, and the overwhelming majority of what was there remains.

The decode: what he overwrote, and why most of it survives

Why a member is not a disk: the misunderstanding at the centre of this, and it is extremely common. A drive that has been part of an array does not hold whole files. It holds stripes — fragments of files interleaved across every member according to a layout described in metadata written at a known position on each disk. That metadata records the member order, the stripe size, the layout and each disk's role. Without it, a member is a disk full of fragments with nothing explaining how they fit.

What creating a partition did: writing a new partition table and filesystem header places a small structure at the very start of the disk — and that is precisely where the array metadata lives. So on two members, the description of the set was replaced. Nothing else was touched, because creating a partition does not write across the whole disk; it writes a header and marks the rest available.

Why that is far better than it sounds: the ratio. What was overwritten measures in megabytes at the front of each 2TB disk. What survives is everything behind it — the actual striped data, untouched and in place. So the loss is the description of the array rather than its contents, and a description can be derived by other means.

How the set is reassembled without it: the parameters are recovered from the members he did not touch, which still carry their metadata and describe the layout of the whole set. Where those are insufficient, the geometry is determined analytically — stripe size, order and rotation are deduced by testing candidate layouts against the images until file content reads coherently across the members. That is what makes this recoverable rather than a guess: a correct layout produces playable media and an incorrect one produces nothing.

Why the media content helps: large audio and video files written contiguously make excellent validation material, because a wrong stripe size or member order shows up immediately as unplayable output.

What must not happen now: nothing further written to any member — no formatting, no filesystem repair on the new partition, and no attempt to rebuild the array on hardware, which would write fresh metadata over what remains. And the drives he did not partition are now the most valuable objects in the case.

On the bench

All members were imaged individually write-blocked on the Atola TaskForce 2, so that every hypothesis was tested against copies. Array parameters were derived from the members that had not been modified, which still carried the metadata describing the whole set — which is why those drives, rather than the repartitioned ones, decided the case. Geometry was then validated against file content rather than assumed, since a correct stripe size and member order produce coherent media and an incorrect one produces nothing. The set was assembled offline from the images.

The outcome

Every member imaged individually, the layout derived from the untouched drives and validated against playable content, and the set assembled offline. 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 who has repartitioned a former array member: drives from an array do not hold whole files but interleaved fragments, described by metadata at the very start of each disk — which is exactly what a new partition table overwrites. But that structure is tiny against a 2TB disk, so what was lost is the description rather than the data, and the description can be rebuilt from members you did not touch.

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.