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.

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

The opening screen shows folders on your computer that are being backed up to the cloud, and you can add new folders or remove folders already in the list if necessary.

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.

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.