ReadyNAS 428 drive not adding to volume

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greenlightcomp wrote:

I am also expecting the total capacity should increase as all drives are now 4TB instead of having some 3TB ones in there, I am using flex RAID.

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With X-RAID it would go up by 1 TB (from 27 to 28 TB).  The resync would happen in two steps (because you would have two RAID groups, due to the original mix of 3 and 4 TB drives).

But FlexRAID is another matter.  You have a 21 TB RAID-5 volume now.  The KB article says it should expand to 28 TB, now that all disks are the same size.  https://kb.netgear.com/23136/How-do-I-vertically-expand-a-Flex-RAID-volume-on-my-ReadyNAS-OS-6-storage-system

Let us know whether that happens.  BTW, the expansion will start after the resync completes (and will look like a second resync from the web ui).  You won't see the capacity increase until that second resync completes.  That will take a while (though not as long as the first resync).

BTW, you really do need the space.  BTRFS doesn't behave well when the file system gets very full (and yours is about 98% full).  After the expansion completes, I recommend running a balance from the volume settings wheel.  That might free up more space.  It might take a while (days) if you've never run one before.  Then perhaps set up a maintenance schedule (also done on the volume settings wheel).  I run one of the 4 maintenance functions every month (cycling through the set 3x over the year).