Source:
http://www.galaxywap.com/2016/07/how-to-protect-and-secure-external-hard.html

Security becomes our first priority especially when it comes to personal datas which are of great relevance to us. Like the external Hard Disk Drive which of course contains alot of personal datas stored in it, is suppose not to be getting access by unauthorized users, remember you can set a desired password to unlock the external drive on any computer.
So follow the guidelines given below line by line, and i assure you'll definitely get to know how to protect your External Hard Disk Drive.
Also read How to hide folder/file on your laptop/PC and make them invisible

How To Protect External Hard Disk Drives With Passwords
Below was mentioned, two methods on how to protect your External Hard Disk Drive with strong security passwords. One of the method can be done based on system’s settings, that is without using a third-party software. While the other method requires a third-party software to run the operation.

Method 1.
Secure Your External Hard Drive With BitLocker:
This is the system settings method and below are steps to help you perform the action.

Step 1. At first, connect your external hard drive to your Windows PC and turn it on. Now click on your Start button and then navigate to Control Panel--> BitLocker Drive Encryption.

Step 2. By default BitLocker will be “Off”, so you need to turn on BitLocker for the drive that you want to encrypt.


Step 3. Now, you will tick on “Use a Password to Unlock the Drive” option there, and you'll see two blank fields, enter the same password in both fields and then click “Next“.

Step 4. Now it's time to save your key somewhere you destined it to be. To do that click on “Save the Recovery Key to a File”, and then select any location you wish to save your key for future reference and then proceed by clicking on next button.

Step 5. Now click on Start Encryption option and wait for the process to complete (this can take time depending on the drive size) time to complete. Once finished.
That’s it! you are done, now your drive will always request for a password to access files inside it.

If this doesn't satisfy you, see the other method
At http://www.galaxywap.com/2016/07/how-to-protect-and-secure-external-hard.html