I was able to access my WD MyBook (that contains a 1Tb drive) through the network connected on the USB port of the NAS. The problem that I've encountered was that it took about 5-10 seconds to enter a folder or to start copying files, the transfer speed was similar to that of my NAS (about 12-15Mb/s) on large files, but it took forever for small files since i had the 5-10 seconds delay for each file.
As a note, the external drive was formated as NTFS (as I'm using as a backup drive for my Win7 PC) so that might have been the problem with the browsing/reading delays.
Anyways, I don't intend to use it like that, it was just a test.
I'll have to try again with another drive formated as FAT32 or Ext3 and see if that changes anything, only that now i don't have a spare drive at the moment and i need all the data stored on the 1TB drive.