Upgrade HDD or get new device

If you go with a new unit, the 2TB drives would certainly be more cost effective than the 1TB. If you went with 3x2TB Samsung HD204UI drives (on the HCL) then it would be $360 for the three vs the $408 for the 4x1TB drives I mentioned above and would have 1TB more storage space as mentioned. Then you could use the NV+ as a backup, and in a year or so switch it over to 2TB drives as well. Or you could update your current, complete backup (of course you have one) and then do the swap as StephenB mentioned.

Yes, the trusty old NV+ is a bit slow, but unless you are copying large files, it's not that bad. In a backup situation it's not critical.

We can offer suggestions and offer opinions, but you have to make the decision. I know the best decision I ever made from a peace of mind status is when I went to two ReadyNAS units, the second backing up the first every night starting at midnight. There have even been occasions when I have messed up a file and simply pulled the previous version down onto the main NAS from the backup and start over. One rule for security is to never, ever work with a file on the backup unit. If you mess it up too, you are SOL (Straight Outta Luck).