NAS Volume Degraded or Crashed? What to Do First

A NAS (network attached storage) device usually stores data on several disks combined into a RAID array, with a volume and a file system on top. When it reports a degraded, crashed or missing volume, the problem could sit in any one of those layers. The right response depends on which layer failed, and the device's own repair options do not always tell you that.

Degraded normally means the array has lost redundancy, for example one disk in RAID 5 has failed, but the data is still readable. Crashed or failed normally means the array has lost more than it can tolerate, or the device can no longer assemble it. A volume that is missing, read-only or shows as uninitialized can point to a damaged file system or lost configuration rather than to a dead disk. Names and wording differ between vendors, so read the exact message and note it down.

Stop writing to the device. Pause scheduled tasks such as backups, snapshot replication, indexing and scrubs where you can. If the data is the only copy, copy off the most important files while the volume is still readable, choosing the files you cannot replace first, and stop if the disks begin to error or the device slows to a crawl.

Write down the make and model, the RAID type, the number of disks and their slot order, and the exact error text. Photograph the screen and the drive bays. This costs nothing and is very useful later.

Do not remove and reinsert several disks to see what happens, because changing the order or forcing a disk back can make the array metadata inconsistent. Do not run a repair, a file system check or a rebuild before you know which layer has failed. Do not create a new volume or reset the device to factory settings, since either can overwrite the structures that describe where your data is. And do not replace a failed disk with a new one and start a rebuild if other disks are showing errors, because a rebuild reads every remaining disk in full.

The array records which disk holds which part of the data and in which order. If disks are pulled out and put back in a different order, or moved to another device, working out the original layout becomes harder. Label each disk with its bay number before removing anything. If you have to take the disks out, keep them in the same order.

Disks in a NAS are often bought together and worn together. When one fails, others may be close behind, and a rebuild puts heavy load on them. Check the health and error counters of all the disks before you start anything, and if more than one shows trouble, treat the array as fragile.

If files have changed names or cannot be opened, the cause may be ransomware rather than a hardware fault, and the steps are different. Disconnect the device from the network and do not delete anything. Some NAS devices also keep snapshots that may survive, so do not delete them or trim their retention.

Ask a specialist before repairing when the data has no other copy, when more than one disk has failed or shows errors, when the message mentions a crashed volume or a missing pool, or when a first repair attempt has already made things worse. Work is normally done on images of the disks, so that the original disks are read as little as possible. No one can promise a result before looking at the disks, and we say plainly what appears realistic after an assessment.