I have a 3TB Seagate Backup Plus Desktop Drive. How can I set a password on it?
BitLocker is the best if your system has TPM support or TrueCrypt is the best alternative.
I'd strongly suggest you check out TrueCrypt. For security of external (and internal, for that matter) storage devices, you really cannot beat it.
You can have a look at some of the suggestions mentioned in below link -
Can I set a password for my external hard drive and backups?
You can use Truecrypt to protect your data.Makeuseof also has a guide for it -
TrueCrypt User’s Guide : Secure Your Private Files
How To Password Protect Your USB Stick: 3 Easy Ways
What is the BitLocker To Go Reader?
mitracker file secure