RAID Array Failure
A RAID array is designed to survive a single disk failure — but the recovery process itself is where most RAID data actually gets lost, usually at the hands of a well-meaning attempt to "fix" it.
What's actually happening
RAID spreads — and, depending on the level, duplicates — data across multiple physical drives according to a specific layout set when the array was created: the RAID level (0, 1, 5, 6, 10...), the order the disks were added in, the stripe size, and sometimes vendor-specific logic on top. When a disk fails, drops out, or the metadata describing that layout gets corrupted, the array as a whole becomes inaccessible even though most of the actual data is still sitting untouched on the surviving disks. The problem is knowing how to put it back together correctly.
How recoverable this really is
It depends which of three quite different situations is actually happening, and they call for different handling:
- Logical failure — the disks themselves are fine, but the array won't mount, due to accidental reconfiguration, a corrupted superblock, or the controller losing track of the layout.
- Single physical disk failure within what the RAID level tolerates (one dead disk in RAID 1/5/6/10).
- Failure beyond what the level tolerates — multiple simultaneous disk failures, an interrupted rebuild, or any failure on RAID 0 — where the array has lost its safety margin and reconstruction has to be handled very carefully.
The first is sometimes manageable with care; the second is workable but unforgiving of mistakes; the third has essentially no room for error.
The general approach
Recovery starts by determining the original array parameters — level, disk order, stripe size, offset — either from surviving metadata or, if that's gone, by analyzing the raw data on each disk for the repeating patterns RAID striping leaves behind. From there, the data stream is virtually reconstructed without altering the source disks. This is normally done by imaging each drive individually first and working only from those copies afterward. Any disk that's physically struggling needs to be imaged with tools tolerant of read errors before reconstruction is attempted at all, because rebuilding an array means reading every disk repeatedly — and a failing drive often won't survive that many passes.
Don't let the controller or NAS "rebuild" or "repair" the array on its own once something has clearly gone wrong, and don't try a different configuration to see if it starts working. Both are extremely common ways a recoverable array becomes a genuinely unrecoverable one — a rebuild writes new data across the whole array based on assumptions that may simply be wrong.
Practical, step-by-step guides for specific recovery software will be added here as separate linked articles.
This page describes the general situation, not a guaranteed fix for your specific case. If you'd rather have someone experienced take a look before you try anything, see what a hands-on evaluation looks like.