A legacy volume is a volume created in QTS 3.x or earlier, before QTS had storage pools. A NAS will contain legacy volumes in the following situations:
A volume was created on a NAS running QTS 3.x or earlier, and then the NAS was updated to QTS 4.0 or later.
A volume was created on a NAS running QTS 3.x or earlier, and then the disks containing the volume were moved to a different NAS running QTS 4.0 or later.
You can use legacy volumes for data storage, but their behavior and status will not be consistent with other volume types. They also cannot use newer QTS features such as snapshots.
QNAP recommends replacing legacy volumes with newer volumes. To replace a legacy volume, back up all data, create a new thick, thin, or static volume, and then restore the data to the new volume.