RN104 degraded volume resyncing - Less than 10% of volume data's capacity is free

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Isa28 wrote:

(1) first to backup the data to another NAS after the sync has finished

(2) then replace both 3TB discs with larger discs (I am thinking of 6TB)

What will happen then?

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First, confirm that you are in XRAID mode - there should be a green stripe on the XRAID control on the volume tab.

After the backup, you hot-insert the first new drive (removing and replacing the disk with the NAS running).  The NAS will detect the new drive, and resync.  The resync is reconstructing the 3 TB of data that was on the old disk from the remaining ones. It will then expand your volume by 1 TB (from 10 TB to 11 TB). 

Note these sizes are in "classic" TB (1000*1000*1000*1000 bytes).  The NAS (like windows) uses TiB (1024*1024*1024*1024 bytes), but uses TB as the label.  10 TB is about 9.09 TiB; 11 TB is about 10 TiB.

When the resync completes successfully, you hot-insert the second drive in the same way.  The NAS will resync again - reconstructing the 3 TB of data that was on the original second drive from the others.  

When that second step completes, the NAS will automatically expand the volume by 3 TB from 11 TB to 14 TB (~12.7 TiB ).  This might happen in two steps (11-12, and then 12-14).

The system might prompt you to reboot the NAS before it expands the volume- either after the first resync or the second.

Since you have a backup NAS, you could alternatively switch to that after the backup, and do a clean factory install (either from the boot menu or the web UI).  Then reconfigure the NAS and restore the data from the backup NAS.  The disadvantage is that your data won't be available until after you restore it - so you'd need to use the backup NAS as primary while that's being done.  But if you have a gigabit network it could finish a little faster, since creating a volume from scratch takes about the same time as a single resync (and you need to do two).  It also gives you a completely clean file system.