D-Link DGS-1016D and ReadyNAS Duo

I don't have any experience with the D-Link rack mount switches, but I have a DGS-2205 desktop switch (apparently now discontinued) that I had in service with my NV+ and it worked well with no problems, until I added my first x-86 class ReadyNAS (NVX Pioneer) and it was fine as long as I was copying file onto the NAS, but disastrously slow transfer when copying files off (less than 1KB/s). Netgear technical support was convinced that I had a bad chassis, and was preparing to issue me an RMA. At the suggestion of this forum, I tried a direct connection and suddenly all the problems disappeared. Needless to say the DGS-2205 is now in my closet, replaced by a Netgear GS-108.

You might want to take one of the client stations that is experiencing this problem and connect the Duo directly to it to see if it could be the switch.