Data Recovery Case File · NAS & Network Storage · Two Disks Behind One Letter

Formatting a Dual-Drive External Set to RAID

His enquiry was short and contained one word that changes the whole job. "I have accidentally formatted my external 4TB dual-drive unit which was set to RAID. It contains image and artwork files." A format is ordinarily among the more recoverable accidents in this archive, because a quick format replaces the index and leaves the contents physically in place. That remains true here. But an external that presents one drive letter over two physical disks adds a layer, and it changes both what has to be reconstructed and what he must avoid doing next — because the most tempting remedies on a unit like this are the ones that finish the job the format started.

The decode: what one letter over two disks means, and what a format did to it

What the unit actually is: two ordinary hard disks in a single enclosure, combined by a controller into what the computer sees as one volume. Consumer units of this kind are commonly configured striped — combining both disks into one large volume for capacity — which is why a 4TB unit is attractive. That configuration carries no redundancy: every file is split into chunks written alternately across both disks, and neither disk holds a complete anything on its own. Some units are configured mirrored instead, which is a different and more forgiving situation. Which one he has is the first thing to establish, and it is not always what the box suggested.

What the format did: the same thing a format does anywhere. It wrote a fresh, empty filing structure at the level of the combined volume and marked the space as available. It did not erase the contents, and it did not un-combine the disks. So his artwork is still physically present, distributed across both members exactly as it was, sitting beneath an empty index that claims the space is free.

Why reconstruction is a step longer: recovering from a format on a single disk means finding the previous filesystem and rebuilding it. Here that filesystem does not exist on either disk — it exists across both, in an interleaved pattern. So the array's original geometry must be derived first: which disk came first, what chunk size was used, and where each member's data begins. Only once the volume is reassembled correctly can the previous filesystem be located beneath the new one and rebuilt. That is ordinary work, but it means both disks are needed and both must be imaged.

What must not happen now: three things, and all of them look like fixes. Do not let the unit rebuild or resynchronise — on a mirrored configuration that copies the newly-empty state across both members and destroys the surviving copy. Do not re-create or re-initialise the array to "put it back", because that writes fresh array metadata over the configuration a reconstruction reads. And do not format it again or write anything to it, since every write lands on artwork. The unit should be powered down and both disks removed, labelled by bay.

Why the material helps: image and artwork files carry strong internal signatures, so where structural reconstruction cannot fully succeed they are recoverable directly by content. For a designer, though, folder structure and filenames matter as much as the files, so reconstruction is attempted thoroughly before any carving.

On the bench

The unit was powered down before anything else, since a dual-drive enclosure left running after a format is one automatic resynchronisation away from a much worse position. Both disks were removed, labelled by bay, and imaged write-blocked on the Atola TaskForce 2. The array's original geometry was then derived from the images by testing candidate layouts against the previous filesystem's own structures — member order, chunk size and offsets as they were before the format — and the volume reassembled offline from the copies. The earlier filesystem was located beneath the new empty one and rebuilt, returning folders and filenames, and his files were verified by opening.

The outcome

Both members imaged, the original geometry derived and the previous filesystem rebuilt offline from the copies. 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 formatted a multi-disk external: a quick format replaces the index and leaves your files physically present, which is the good news; but a unit presenting one drive letter over two disks stores everything interleaved across both, so the array's original geometry has to be derived before the previous filesystem can even be located — which means both disks are needed; and the dangerous remedies are the obvious ones, so do not let it rebuild or resync, do not re-create the array, and do not format it again. Power it down and label the bays.

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.