external drive network security
If the admin has access to the computer remotely while you are logged in then there isnt much you can do about it. As soon as you connect the drive to the computer (encrypted or not - as you said) the drive will become available to anyone who can login to the computer. 

The only sure fire way is to bring in a laptop.

Its also worth checking if your company has an IT policy, it may be against the rules to try to 'hide' a connected device from the systems admin or it might be against the rules for the admin to view personal drives connected to machines.