Can I hide a drive from the filesharing network, even from admin acct?
Hmmm... interesting question. I haven't tried this personally but it looks like you can edit /etc/fstab and add the UUID of the disk with the 'noauto' flag:

http://www.radiotope.com/content/os-x-hiding-and-mounting-partition

I was going to propose changing permissions to prevent the other user from having access to the files. But, as tubedogg mentioned, if the user is an admin they can undo anything you've done.

Perhaps it's best to eject it when you're done if you're concerned about other admins having access to the data?