Odd drive sharing problem

[Unleashed]

Okay guys, this is the breakdown of my little home network.

Server running Ubuntu has three drives:
80GB for OS (IDE internal)
250GB for storage (IDE internal)
500GB for storage (USB external)

My computer dual boots Windows XP and Ubuntu 7.04 (Feisty).

Now, the problem:
I'm sharing both the 250 and the 500gb drives on the network. My XP installation can pick both of them up, with read and write, no problems.
However, if I go into Ubuntu, I can mount the 250gb fine but the 500gb says permission denied!

Anyone got any ideas? I'm stumped...