just in case another of the 16,850 or so old farts like me who have visited this forum over the years should stumble upon this thread and find no answers until they finally get to this one:
yes I tried all of the above. No I don't have an old driver -- my scenario: went to a new ISP and had to plug everything into a new modem/router. All the other peripherals worked fine -- (including the DiskStation NAS) but 2 days of headscratching could not get the D-Link DNS-321 to be recognized by Windows 7 or the Easy Search utility.
What finally did it: - I routed the NAS from the new wifi router to an equally old gigabit switch (D-Link DGS-1008G) causing the speed of the NAS to step down to 100 Mbps (instead of gigabit 1000 Mbps). I don't know why that worked - but it did - so there. YMMV. Cheers.
PS: once I could access the online configuration utility for the NAS, I noticed in the LAN settings that 1000 Mbps speed (as an option) was greyed out. So I'll keep using this thing for a while longer at a slower speed (just like me)