windows 8 - How to encrypt an external hard drive so a password is required only on computers other than my own?

I want to encrypt my external hard drive but I only want it to require a password if being accessed by a PC other than mine.

This is because I have Backblaze set-up on my own PC and if Backblaze can't get passed the encryption, it won't back up the data on the external drive. But if someone were to take the drive and try plugging it into another PC I would of course want the encryption to work so they could not access it.

I'm using Windows 8.