Ok I have now solved this, this is what I did (with the help of very good friend)
Use WinSCP to edit /etc/samba/smb.conf.template
Go down the list and remove the line:
invalid users = root
Then add this lot at the bottom of the list:
[Downloads] <----- [The name you want to give the folder on the share]
path = /mnt/exFAT128GB/Downloads <-----/mnt/The folder name and path of your USB drive
valid users = root
read only = no
create mask = 777
force create mode = 777
security mask = 777
force security mode = 777
directory mask = 2777
force directory mode = 2777
directory security mask = 2777
force directory security mode = 2777
The final step is to set an SMB password using Putty. Login using the steps mentioned.
smbpasswd -a root
Type a password of your choice (or use the same root password as I did). Confirm the password again.
Type exit to log out. Then reboot the router.
You should then go to Windows Network and see something like this:
Access the Downloads folder (or whatever you named it), a login box will appear, enter username as root and your password. You will now have write access to the folder.