RAID6 resync after drive failure: Data Degraded message

Replacing the drive will add no space, it will add a second layer of redundancy.  RAID redundancy CAN save your data from failure, and RAID6 will do better at that that RAID5 ( which degraded RAID6 effectively is), but there are many types of failures that RAID cannot overcome.  You should really think of RAID more for maintaining maximum AVAILABILITY than backup.  That is, for many failure types, such as a single drive failure, you don't have to take the NAS offline to restore data from a backup.

Having lost one drive, that could mean your others are also approaching an age at which failure is more likely.  It all depends on whether they were put in the NAS at the same time.  RAID re-sync puts  a  lot of stress on the drives, so another could fail during the re-sync.  But right now, you can lose another drive and not lose the volume.  If you wait for another failure, you will lose the volume if yet anther fails during re-sync after you replace the second failed one.

If, after reading this, you still just want RAID5 instead of RAID6 and you want the "degraded" state to go away, see RAID6-to-RAID5-without-volume-re-creation-is-possible.