I am surprised to see that the transfert through FTP gives not more that 15 Mo/s through a gigabit link.
It looks like the hardware is limiting, since I have checked things like the following :
Transfert starts with 50+ Mo/s then slows to 15 Mo/s as in normal procedure when packets are sent faster than the receiver can handle
ARM proc is always between 50 and 100% of usage whith respectively 15 and 30+ Mo/s
Thus I am thinking, maybe the hardare of the network attached storage WD My Book World Edition is not able to withstand an offered flow rate of 100Mo/s.
Finally, please let me know if this issue can be solved.