How much does a drives RPM make a difference?

@zamboni, Thank you for the 3TB suggestion, but the NV+'s inability to go beyond 2TB is somewhat of a non-starter on that one. I don't want to invest the money right now in a new NAS. I was really hoping the extra space from the 2TB drives would last me at least another year or two. Perhaps then I would be in a position to buy a new NAS unit. Incidentally, I have a 2-bay D-Link NAS unit that I am not using (I bought it before my first NV+ and so much prefer my NV+ that I scrapped the D-Link) and could fill it with two of the 1.5TB drives if I needed the extra space.

@dbott67 and PapaBear, thank you for the information. The lower power and lower heat of the 5900 RPM drives is particularly attractive as my NV+ units are in a room with a southwest exposure (I live in Arizona) and, thus, gets quite warm at times. Anything I can do to keep the heat down (as long as I'm not taking a noticeable performance hit) is appreciated. PapaBear, I also like your suggestion of using a dock (I already have two but don't use them much) and the existing 1.5TB drives as back-ups. I might also want to keep one or two of the 1.5TB drives as spares to for the second NV+ (the one I am not backing up). Incidentally, fwiw, I have the second NV+ (the one I use as for backing up the first NV+) set to only turn on at night when it runs the nightly back up of the first NV+.