I recently had a need to turn on Bitlocker encryption (in Windows 8.1) for a 1.8 TB external hard drive. After three workdays (more than 24 hours), the drive was claiming it was only 5.2% completed with the encryption step. I searched all over and didn't find an answer, but a colleague recommended plugging the usb drive into a USB 3.0 compliant port, vs. a USB 2.0 port. I had that option available on my machine, and as I'm writing this (about three hours later), the drive is about 67% encrypted. It's working!