[Hampshire] Can't write to NAS

Hi all

I'm having a problem writing to one of my NAS devices with my Ubuntu box. Here's the setup:

2 PCs (one XP (marsh) and one Ubuntu (thomson))
2 NAS's (cnfs and cnfsusb).

marsh (the XP box) can read and write to both NAS's.
thomson can write to cnfs, but not to cnfsusb

So one would think that it must be something to do with the permissions on cnfsusb - but the user I login as on XP is the same user I login as when I mount cnfsusb on thomson. I've set the permissions on the mount point such that any user can write to it - but when I mount it - these permissions are mysteriously reset back such that only root can write to it. Trouble is - even trying a simple

sudo uname > /mnt/cnfsusb/fred

results in a permission denied. The mount command I'm using is exactly the same to mount cnfs as cnfsusb.

So I'm a bit stuck. Any ideas?

Cheers
Rob