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Synology RAID 5 (4 × Seagate 3 TB) — RAID recovery case

Synology RAID 5 offline with two of four drives failed — imaged every member, rebuilt the array virtually, full recovery.

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Full recovery Case DDR-2026-0003 · Completed July 17, 2026
DeviceRAID
ManufacturerSynology
ModelRAID 5 (4 × Seagate 3 TB)
Capacity9 TB usable
InterfaceSATA
OutcomeFull recovery

Serial numbers and any customer-identifying details are deliberately omitted.

What came in

A Synology NAS running a four-drive RAID 5 had gone offline and the volume was no longer accessible. Two of the four member drives had failed — one more than a RAID 5 can survive.

What we found

The array was built from Seagate Barracuda 3 TB drives: three ST3000DM001 and one slightly newer ST3000DM008. Two members had failed, and one of them was reporting only a few gigabytes instead of its full 3 TB — a capacity/translator fault rather than a straightforward mechanical failure. We determined the array parameters as RAID 5, left symmetric backwards, with a 64 KB block size and a parity delay of 1, carrying an EXT3/4 file system.

What we did

Nothing was rebuilt in the NAS. Each member drive was imaged individually so the originals were never written to, and the failed members needed drive-level work before they would give up a usable image — including the one misreporting its capacity. With images in hand we reconstructed the array virtually using the determined geometry (left symmetric backwards, 64 KB blocks, parity delay 1), verified the layout was correct, and extracted the EXT3/4 file system from the reconstructed volume rather than from the drives themselves.

Seagate Barracuda ST3000DM001 3 TB drive labelled by hand as drive 2 and marked "Failed", with the serial number redacted
One of the two failed members, marked at intake. A RAID 5 tolerates exactly one drive loss — this array had lost two, which is why it went offline rather than running in a degraded state.
Seagate Barracuda ST3000DM001 3 TB drive marked by hand as drive 3, noted as showing only a few GB, serial number redacted
The second problem drive, annotated by our technician: it was reporting only a few gigabytes instead of its full 3 TB. That is a capacity/translator fault, and it has to be addressed at the drive level before the member can be imaged properly.
Seagate Barracuda ST3000DM001 3 TB array member drive label with the serial number redacted
A healthy member. Three of the four drives in this array were ST3000DM001s — a model with a well-documented history of high failure rates.
Seagate BarraCuda ST3000DM008 3 TB array member drive label with serial number, WWN, PSID and QR codes redacted
The fourth member, a newer ST3000DM008 from 2017. Mixed models and manufacturing dates across one array are normal in a NAS where drives have been replaced over the years — but it also means the members age at different rates.

Outcome

Full recovery.

Two points worth drawing out of this one. First, three of the four drives were Seagate ST3000DM001s — a model with a well-documented history of high failure rates — so two of them failing in the same array is less bad luck than it looks. When every drive in an array comes from the same model and era, they tend to wear out together, which is exactly how a single-drive-tolerant RAID 5 ends up losing two at once. Second, and more important: the array was never rebuilt. With two members down, hitting "rebuild" writes across the surviving drives and is the most common way a recoverable array becomes an unrecoverable one — we cover that in RAID rebuild vs. data recovery. Imaging every member first and reconstructing the array from the copies is what made a full recovery possible here. More on this scenario in RAID 5 with two drives down.

Every device is different. This is what happened with this specific drive — it isn't a prediction about yours. Two drives with identical symptoms can have very different causes and very different outcomes, which is exactly why we diagnose before we quote, and why we don't publish a success rate.

Dealing with something similar? See our raid data recovery service, browse more cases in the case log, or read about what makes recovery hard.

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