If you have more than 4TB on that NAS it is going to be hard (and slow if not impossible) to do without first copying your data elsewhere. In factm, doing that and verifying those backups is a good first step regardless of any other actions taken.
Next you have at least these choices:
- Pull one drive (RAID5 keeps working with one drive less, though there is no more reduncancy).
Copy all data to the 4TB drive.
Wipe the 3 disk array.
Create a new array and copy the data to that.
Pull spare drive for resuse.
- Or backup all data and verify that backups work.
Reconfigure array. e.g. with a 3 disk RAID5 or a 2 disk mirror.
Copy all data back.
is it possible to use just 3 disks or 2 disks?
It is possible to use just one disk. JBOD. No redundancy at all. 4TB useable.
You can have two drive each with a copy of all the data (mirror). That way one drive can fail and you still have all your data. This also speeds up reads. 4TB useable.
You can use three drives in a 3 way mirror (RAID1E). Not that common unless read speed is very important. 4TB useable.
Or you can use 3 disks in RAID5. That way you still can loose on drive and keep all data. read speed is fine but writes (esp small ones) may suffer. Useable space is 8TB.
For more details about RAID and their levels please see this post on our sister site.
Also, consider that RAID is not backup. So you do want awell tested [off-site] backup. And that means you can just wipe the array and restore from backup.