If you don't want to hide the drive or the files inside, then you cannot reliably secure an external hard drive. You can use the ACLs of NTFS (assuming that you format it with NTFS) and as long as its connected to your computer and you are logged on as you, you will have full access to it. However, if you log on as somebody else, they won't have access to it.
Unfortunately, NTFS security can be easily overwritten by anybody who has the appropriate rights. To override the rights assigned by a particular computer, you just need to attach the external drive to another computer that supports NTFS, and take ownership of the files and folders.
If you use a 3rd party program to secure it, then the files can be accessed once the external drive is plugged into another computer, unless the 3rd party program hides the files and folders such that none of them can be accessed until 1) the password is entered, and 2) the security program is installed on the computer to which the drive is attached.
The best method of securing with the files will be with a 3rd party application that hides the files and folders until you enter the password.