Some NICS and HUBs don't like talking to each other if they're forced to go full duplex and 100Mbs. In theory, you don't need the full duplex enabled except if you're copying files from A to B and B to A at the same time; It's usually this that screws it up. Apparently it's something to do with the layers and translation when IP's go to MAC address and hardware handshaking... IT all depends on your hub and NICs as well - what make are they?