We setup a QNAP TS451 recently in RAID 5 out of the box with 4x4TB HDs.

It's turned out to be too much space and I'm interested in reclaiming some of the 4TB drives to use elsewhere.

Wondering about the steps / pathway to downsize the total overall size of the NAS (4TB would be fine) AND if it's possible to use just 3 disks or 2 disks?


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:

  1. 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.
  2. 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.

  • Thanks for these suggestions; just to confirm: 1. while the unit is powered off, just remove one of the drives [format it] 2. on the removed drive copy the contents of the NAS 3. Reset the 3 remaining disks in the NAS at RAID5 and copy all data back 4. The drive the data was copied from can now be removed and reused elsewhere Confirm this is good and I'll give it a go - thanks heaps – alexh Dec 19 '15 at 11:33
  • 1a) While powered off is a good idea (though SATA supports hotplug). 1b) If you have an option to degrade a drive or remove it from the array that might actually be cleaner. 2). Assuming it fitt on one drive. And personally I prefer my backup on another system. Both in case of human errors and in case of lightning strike etc etc/ 3) Yes. Please do read the linked article on the write gap with RAID5 and the risks of URE's when rebuilding. RAID 5 is great for two things only: Reading speed and lots of space per disk used. It has drawbacks. – Hennes Dec 19 '15 at 12:30

I would rebuild the raid with 3 disks if you want to reclaim one of the hard drives. You will need at least 3 drives in a raid 5 configuration, 2 drives won't work.

  • True. But could you point out the stept to be taken? Right now it reads as 'wipe all' 'rebuild' 'restore'. I got the impression that the OP was looking for some more details, possibly without backup and restore. – Hennes Dec 18 '15 at 22:40

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