It depends.
If you want to have your personal keys stored in an external HD, you can, but make some more copies to CDs or USB sticks because an HD can be broken.
If you want to make transactions, you can use cold storage but you need a second (always offline) PC. You can read the procedure here:
http://docs.electrum.org/en/latest/it is about electrum and Bitcoin but the procedure is the same in all coins and wallets.
If you want to check the balance you can create a watching-only version of your addresses (under the same link).
I hope I've helped.