Using a NAS is very convenient – data is available from anywhere, multiple users can work simultaneously, and thanks to RAID 5 you also have the feeling of safety, that if one disk fails, you won’t lose your data. RAID 5 distributes data with parity, so it can recalculate missing content. In practice, however, this does not mean 100% certainty. Sometimes things don’t go according to the ideal scenario, and the entire array collapses.

Common causes of NAS RAID array failures

Physical failure of one or more disks – the most common cause of RAID array failure. NAS disks run continuously 24/7 and are subject to higher thermal and mechanical stress. Over time, bad sectors, bearing noise, or electronic issues occur. If one disk in RAID 5 fails, the array should still function, but if a second disk fails, the data is usually no longer accessible.

Sudden ATA or SMART errors – even a disk that appears healthy can start reporting communication errors, so-called ATA errors. Similarly, SMART attributes can reveal bad sectors or unstable platter surfaces. These errors cause the NAS to mark the disk as faulty, even if it is still physically working, and exclude it from the array.

Unsuccessful rebuild after disk replacement – when replacing a disk, the NAS starts a rebuild – recalculating parity and regenerating data. If an error occurs during this process (a faulty new disk, power failure, write error), the rebuild is not completed and the entire array may fail. This was, in fact, the cause of failure of this Netgear NAS.

Failure of the controller or power supply in the NAS itself – it’s not only disks that can fail. A faulty RAID controller, power supply, or NAS firmware can cause the array to become inaccessible. In such cases, the disks themselves are physically fine, but the NAS cannot read them properly.

File system corruption (e.g. Btrfs or EXT4) – even if the RAID is assembled correctly, the file system can still be the problem. A power outage at the wrong time, a failed write, or a faulty disk can leave Btrfs or EXT4 in an inconsistent state. The user then sees no data, even though it is physically present on the disks.

The attempt to increase storage capacity led to its failure

The user of this Netgear NAS decided to gradually replace the original 3TB disks with new 6TB models. He replaced one disk, let the array rebuild, replaced the second disk, let the array rebuild, and so on. The problem occurred when a newly inserted 6TB disk failed. The NAS started reporting disk errors (ATA errors) and the RAID array collapsed.

However, RAID 5 should be resistant to a single disk failure. During later reconstruction of the array, it turned out that another disk had also been excluded from the array. The cause is not entirely clear, since the disk itself passed tests and showed no errors.

During analysis we discovered that each disk contained multiple partitions – system, swap, and two large RAID areas. The original data was still present on the first RAID area (~2.7 TB), while the newly created area on the 6TB disks was incomplete.

ReadyNAS does not work with RAID in a completely standard way. Instead of creating a single RAID 5 array, it uses its own X-RAID technology, which divides disks into several parts (zones). Each zone always matches the capacity of the smallest disk in the array. When a larger disk is inserted, ReadyNAS maintains compatibility by only using the portion equal to the smaller disks. The remaining capacity is separated into a new zone, from which another RAID 5 is created and logically joined with the original one. This is why the 6TB disks showed two RAID partitions – the first matching the original 3TB disks (~2.7 TB), and the second prepared for the expanded capacity. In this particular case, however, the new area was never fully assembled because additional disks of the same size were missing. As a result, extra members appeared in the metadata, and the entire array became inaccessible.

This caused complications during the RAID reconstruction. Additionally, the Btrfs file system was damaged. After excluding the “faulty” disk and recovering the correct Btrfs superblocks, we were able to restore the complete directory structure and recover the data.

How to prevent data loss from NAS / RAID

RAID is not a backup. Replacing disks and expanding the array always carries a risk.

Do not attempt repeated rebuilds with a faulty or unverified disk – the risk of data corruption increases.

Use high-quality and matching disks, ideally those recommended by the NAS manufacturer.

Regularly back up important data outside the NAS – to another disk, cloud, or external storage. Never rely on the NAS as your only backup.

If the NAS fails, contact data recovery specialists – amateur attempts at home can worsen the condition of the disks or the entire array.

RAID 5 in NAS may seem safe, but even a single mistake during disk replacement can cause complete data inaccessibility. In this case, the problem was caused by a failing new 6TB disk that stopped the rebuild and corrupted the array’s consistency. Thanks to specialized procedures, however, we were able to recover the data from the Netgear ReadyNAS.