Well, if you want to copy the files on the RDM to a new VMDK there is a few options.
1.) You could P2V the system and take the drive that way. Just make sure you don't resize the drive on the P2V so you can do a sync job after the initial copy. So even if the copy takes 2-3 days to finish, you can run a sync after and it will juts sync the changes
2.) If that is to disruptive or doesn't work for you, you can use Robocopy which is built into Windows and copy chunks of data at a time then sync it. Robocopy is a differential copy so after the initial copy it will only take the changes. We use it a lot for large file server moves. We take one share at a time and have the Robocopy scirpt running every hour or so to sync the changes then when we are ready to cut over we take one last sync then break the share and switch over. If you need a hand with a Robocopy script let me know and I can write one for you really quick.
3.) Backup the data with your backup software then restore it to the new location.
These are probably your best options. The reason why I asked if it was an RDM as if it was a RDM from the SAN and not USB you could just move it, however it doesn't seem like that's the case. In all reality 3TB is a fair amount of data to move and since your doing it through USB that will be your bottle neck that you will just have to work around.
Anyhow hope this has helped.
