Think this is similar to when you sometimes try to delete a directory and it won't let you, with the spurious claim that the directory is in use or similar. With this in mind, are you moving or copying the data to your new drive? If the former, try to copy it - then take the opp to verify the new against the original (size, no of files etc) and then delete the original directory as a separate operation. That should do it.
Am just about to buy another ext drive in the next 10 mins or so, oddly enough. I already have a 160GB ext but have filled it up with music collection and photographs I have taken. Was a bit shocked, I can tell you... Mind you, I have found a 300GB Ext Firewire drive, for just £105 so am going to go for that.
Conan