That will work yes, although if anyone else on the system has admin rights they can take ownership of any filesystem resource (ie. the drive, in this case) and add themselves to it's userlist.
However, in a family situation the probability of having anyone competant enough to do so is minimal.
Another option is a PGP Volume. You can't hide it in any other way than what's allready been mentioned, but you can't access it without a password (and the right decryption keys).
This does, of course, require that you get a copy of PGP from somewhere (your favourite retail website, obviously...).