What Happens When a Disk Fails in Different RAID Configurations?

Last modified date: 2026-10-29

Applicable Products

All QNAP NAS models

Scenario

A disk failure can make a volume degraded or inaccessible. What happens next depends on the RAID configuration. In particular, Single-disk and RAID 0 configurations provide no protection against disk failure. Consider the risk and maintain a separate backup before using either configuration.

The case that prompted this FAQ involved a two-disk RAID 0 group on a TS-251D. After one disk failed, the volume became unavailable and the installed apps disappeared. Replacing the failed disk alone could not restore the RAID 0 volume or its files.

Disk Failure Behavior by Configuration

Configuration	Protection against disk failure	What may happen if a disk fails
Single	None	The storage on that disk becomes unavailable. There is no redundant copy in the configuration to recover it.
RAID 0	None	Data is striped across all member disks. Failure of any member can make the entire RAID group and its volume inaccessible. Replacing the failed disk does not recreate the missing data.
JBOD	None	Disks are combined without redundancy. Data on the failed disk may be lost, and the combined volume or data on other disks may also become inaccessible, depending on the layout and failure.
RAID 1	One disk	The group may continue operating in a degraded state using the remaining mirror. A second disk failure before recovery can make data inaccessible.
RAID 5	One disk	The group may continue operating in a degraded state. A further disk failure before the group is restored can make data inaccessible.
RAID 6	Two disks	The group may continue operating with one or two failed disks. Failure beyond its tolerance can make data inaccessible.
RAID 10	One disk per mirrored pair	The group can tolerate a disk failure in a mirror pair, but not the loss of both disks in the same pair.
RAID 50	One disk per RAID 5 subgroup	The group can tolerate one failed disk in each subgroup. More than one failure in a subgroup can make the group inaccessible.
RAID 60	Two disks per RAID 6 subgroup	The group can tolerate up to two failed disks in each subgroup. More failures in a subgroup can make the group inaccessible.

These are general failure tolerances, not guarantees that data will remain accessible or that a rebuild will succeed. Actual symptoms and recovery options depend on the NAS model, firmware, storage layout, disk condition, and whether other errors occur.

What to Do

If data is still accessible

Back up important data to a separate destination as soon as possible. Do not treat RAID redundancy as a backup.
Verify that the backup completed and that important files can be opened or restored.
Check Storage & Snapshots for the affected disk and RAID status. Record the NAS model, firmware, disk bay, status messages, and relevant logs.
For a redundant RAID group, follow the NAS interface or QNAP Support guidance to replace the confirmed failed disk and monitor the rebuild. Do not remove another disk while the group is degraded.

If the volume is inaccessible or the configuration has no redundancy

Stop making configuration changes. Do not initialize, format, recreate, or repeatedly rebuild the storage group, and do not replace additional disks.
Keep the original disks and their bay order unchanged. If the data is important and no usable backup exists, contact QNAP Support or a qualified data recovery provider before attempting recovery. Recovery is not guaranteed.
If a verified backup is available, replace the failed disk as appropriate, recreate the storage configuration if needed, and restore the data from that backup.

In the TS-251D case, the two-disk RAID 0 volume became Not active after one disk failed. Installing a replacement disk by itself could not restore the volume or its files. The advised recovery path was to recreate the storage configuration and restore from backup; without a backup, avoid further changes and seek professional recovery assistance if the data is important.

Prevention

Choose a RAID type with suitable disk-failure tolerance for the importance of the data. RAID 1, for example, can tolerate one disk failure in a two-disk mirror.
Keep regular, versioned backups on a separate system or destination, and test that files can be restored.
Consider backup coverage for the NAS configuration and applications as well as shared-folder data.
For RAID 0, Single, or JBOD, plan for the possibility that a disk failure will interrupt access to data; these configurations do not provide disk-failure protection.