How do you password protect an external hard drive? Are there any reliable freewares available for Windows and Ubuntu Linux?
The best way would be creating a encrypted TrueCrypt volume on your hard disk and saving all your data there. Look at this link:
You may also check out an earlier MakeUseOf article about free apps to hide or lock folders.
i'm using wondershare usb drive encryption to password protect my external hard drive. It's shareware, but it works well for me.
Also check out the default software that came along with the external drive. Companies like Transcend sometimes include a folder guard software.