Failed or Interrupted RAID Rebuild
Replacing a failed drive and starting a rebuild is the correct instinct — but a rebuild that fails partway through, or runs into a second disk with problems of its own, is how a single-disk failure turns into the loss of the entire array.
What's actually happening
Rebuilding a redundant array — RAID 5, 6, 10, or similar — means reading every remaining original disk in full, computing the missing data mathematically from parity or mirrored copies, and writing that reconstructed data onto the replacement disk. That means every surviving disk gets stressed with a complete, uninterrupted read pass. If the rebuild is interrupted — power loss, the replacement disk turning out to be faulty too — or, worse, a second original disk hits a read error partway through, the process can leave the array in a state that's neither the old configuration nor a complete new one: an inconsistent hybrid that generic tools struggle to interpret correctly.
How recoverable this really is
Genuinely one of the harder situations in this catalog, precisely because a partially completed rebuild isn't a clean failure state — different parts of the array may reflect different points in time, and figuring out exactly how far the rebuild got before deciding what to trust is itself a significant part of the work.
The general approach
Recovery starts by imaging every disk exactly as it currently stands, including the partially rebuilt replacement, then carefully working out which portions of the array reflect the pre-rebuild state and which reflect rebuilt data, before attempting any unified reconstruction. This kind of forensic reconstruction is well outside what any consumer or NAS-vendor rebuild tool is designed to figure out on its own.
The moment a rebuild fails, stalls, or throws errors partway through, stop it — don't retry, don't restart the NAS, and don't let it attempt the rebuild again automatically. Each additional attempt reads and writes over the exact disks holding the only accurate record of how far the first attempt actually got.
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.