How do I add a 4th disk on my RN214 that I originally set up with only three?

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dave__ wrote:

 Now I may be jumping the gun as it is still syncing though it still says 8+T.    But it says over nine hours to sync.  Is that normal?

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If it is still syncing you are jumping the gun.  So wait for that to complete.

Syncing requires accessing every sector on all four disks at least once, so it does take a while (e.g., the long time is normal).

The capacity rule is "sum the disks and subtract the largest".  That will give you TB, so you'll need to convert it to TiB.

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dave__ wrote:

then did a factory reset.  

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That step wasn't necessary.  All you needed to do was hot-insert the new disk.  If it was already formatted, you'd also have needed to format it on the NAS volume screen.