Does your computer have a drive letter you can't explain? Here's one possible explanation.

The D: drive is usually a secondary hard drive installed on a computer, often used to hold the restore partition or to provide additional disk storage space.

I’d search YouTube for instructions for your specific drive. Once you have the drive out, you’ll need a dock to connect a bare drive to your computer via USB.

That's it! OS X will perform its first backup (during which you'll want to keep your computer on), and from then on it'll back up in the background with no work required on your part.