New, used, refurb, every NAS has to have a backup. Usually a big USB3 drive. Kept out of thieves' view.I wouldn't recommend buying refurbs due to the much shorter warranty you are likely to get it on it.
If you do get a refurb, it is even more important than otherwise that you backup any important data primarily stored on your NAS as if the NAS has issues you may be on your own to try to get your data back or have to resort to expensive 3rd party data recovery.
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