From your proposed strategy, it sounds like you are not presently maintaining a backup.
Wrong. I do have backups but i only keep backups of things that are important to me (photos, etc) (backup disks are 1tb). I don't consider every movie and Iso as that important to keep a backup of each of them. If they get lost by a "disk crash", so be it. But i also don't want to lose them on purpose just because i wanted to upgrade the NAS.
And even if i had a backup of the complete 5Tb of the old NAS it still would take ages to copy them to the new NAS. <<= That was the point of my posting, JavaLawyer. It takes hours, when it could be a matter of seconds.
It's utter bs that you can't swap the drives from the older NAS to the new one, especially when the new NAS is from the same manufacturer and even the same product line. It's a consumer product, not for professional use, so there should be an easier way implemented.