Specifying which computer's iTunes your iPod syncs with

I have one iPod and 2 Macs. I have iTunes libraries on both computers, but I only want the iPod to sync with the music library on one of them. I need to be able to use the iPod in disk mode with both computers -- my problem is that the iPod wants to sync with both iTunes libraries when connected.

I can set the iPod to 'update manually', but isn't there a way to set the iPod and/or the computers so that it updates automatically with one computer and not at all with the other computer?

 

I tried that, but the auto vs. manual setting seems to be a setting in the iPod, not in the computer. If I set it to either, it works that way on *both* computers. Ironically, I can't even change the setting to manual on the one computer unless the iPod's connected, and by then it's already doing the (unwanted) update.

Isn't the iPod supposed to differentiate between computers? Seems to me I connected my iPod to another computer once and I got a message that said something like "this iPod is set to update with another computer. Do you want to update the iPod with this computer?", or something like that. But I didn't get that message the first time I connected my iPod to my new computer.

When I set up my second computer I transferred all my accounts settings, files, etc via firewire from my old computer. Is it possible that the iPod can't tell the difference between the two computers, even though it's supposed to?