Currently Being ModeratedDec 10, 2013 5:35 AM (in response to Owli)
Make sure you are using the correct cables, i.e not crossover cables, although many switches auto sense it may not in this case.
Likewise try fixing the port speed to 100Mbs on both the NAS and Mac, again it could be auto sensing of speed that is the problem.
I know the Mac supports Gb, I do not know what your switch and NAS support. If fixing the speed cures the problem and the units support Gb then try it it with the ports fixed at a Gb.