Failed or Interrupted RAID Rebuild
Replacing a failed member and rebuilding can be normal maintenance when the array state is known and the remaining redundancy is healthy. Once a rebuild stalls, is interrupted, or encounters additional read errors, however, the array should be treated as a recovery case rather than simply rebuilt again.
What's actually happening
Rebuilding a redundant array reconstructs missing member data from parity or surviving mirror copies and writes it to a replacement or spare. The exact read/write pattern depends on the RAID level and controller, but a rebuild can place sustained load on the remaining members. An interruption does not automatically make every array an unusable "hybrid"; the important question is what was written, which members are authoritative, and whether additional errors or configuration changes occurred before the process stopped.
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 normally starts by preserving the current member set, logs and configuration, then imaging the relevant disks — including a partially rebuilt replacement when it may contain unique state. Reconstruction then determines the correct layout and which member or regions should be treated as authoritative before a virtual array is assembled. 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.
For the underlying RAID layouts and failure-tolerance concepts, start with RAID Array Failure. Imaging every disk before attempting any reconstruction, mentioned above, is exactly what professional imaging hardware is built for when one of those disks is also struggling to read cleanly. See Drive Cages & RAID Enclosures for what to check before buying one, if this is prompting a replacement. If the array needs cleanroom-level physical recovery rather than another rebuild attempt, our directory of major physical recovery providers covers the large international networks.
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.