one way is to enable the root user, but before you do, you must respect how much damage you can do while logged in as root.
open Netinfo Manager in your Utilities folder in Applications. go Security > Authenticate..., and enter your password. now go Security > Enable Root User. now you can log out, and log into root. find the drive and return it's permission level back to what you want. now log out, log back in as yourself and return to netinfo manager to turn root off again.
root is the highest level of user, and can do anything, mostly without warnings. root is not recommended for everyday use as you can very easily screw stuff up.