upper capacity
There may be an upper limit, but on Amazon there is a 72 TB version that would have to come with at least 18 TB drives. If 18 TB is fine, 20 TB is also probably fine, but I couldn’t find any reports by people saying they’d loaded 20 TB drives into theirs without issue.
procedure
You could also clone them yourself, but you’d want to put the NAS into read only mode or take it offline first.
I think cloning drives is generally faster than rebuilding them in RAID, as well as easier on the drives, but my personal experience with RAID is very limited.
Basically, what I’d do is:
- Take the NAS offline or make it read-only.
- Pull drive 0 from the array
- Clone it
- Replace drive 0 with your clone
- Pull drive 2 (from the other mirrored pair) from the array
- Clone it
- Replace drive 2 with your clone
- Clone drive 0 again, then replace drive 1 with your clone
- Clone drive 2 again, then replace drive 3 with your clone
- Put the NAS back online or make it read-write again.
In terms of timing… I have a Sabrent offline cloning hub (about $50 on Amazon), and it copies data at 60 Mbps, meaning it’d take about 9 hours per clone. Startech makes a similar device ($96 on Amazon, that allegedly clones data at 466 Mbps (28 GB per minute), meaning each clone would take 2.5 hours… but people report it being just as slow as the Sabrent.
Also, if you bought two offline cloning devices, you could do steps 1-3 and 4-6 simultaneously, and do the same again with steps 7-8.
I’m not sure how long it would take RAID to rebuild a pulled drive, but my understanding is that it’s going to be fastest with RAID 1. And if you don’t want to make the NAS read-only while you clone the drives, it’s probably your only option, anyway.