External HD connected to BT Home Hub - Cant transfer files to HD

I have just got a new BT Home Hub which is essentially a wireless router with a USB port. I have connected my WD 250GB external HD to it and have been able to connect to it using SMB in Finder.

However the problem that I am facing is that I cannot transfer any files to the external HD. I get an error saying "I do not have sufficient privileges for some of the items". I have checked the permissions and everything seems to be fine. On the other hand, there are no problems when I copy a file from the external hard drive to my desktop.

Also when I connect the external hard drive to my Mac USB I have no problems. Therefore I am unable to figure out whether this is a Mac problem or a router problem.

Can anyone help?

Thanks