Fastest way to migrate DUO RND 2150?

Your only problem would be finding a good NV+ since they have been out of production for almost a year. There are still some left in spots in the retail market where they can be found, but at a high price. Used ones are plentiful on eBay. For the money, I agree with StephenB that you would be better off with an Ultra/Pro which is much faster. While it would require additional drives, once you had the data copied over, you could use the existing Duo (v1) to serve as a backup target for the new NAS. You could go with the new Duo v2, which is ARM based, and with the newer versions of Frontview supports the NFS/rsync backup routine to backup NAS to NAS. You can get a Duo v2 and two 1TB disks for less than I have seen of the older NV+ (v1) units listed. (And the new Duo is far faster).

Edit Update: Corrected NV+ version in last line from (v2) to (v1). The NV+ v2 is the newer NV+ version and is written without () because the v2 is part of the name. To distinguish between the two, most members that post on the forum add the (v1) after the NV+ with () because it is not actually part of the original name.