It's probably a system recovery partition if I had to guess, what you can do is open diskpart in a command line and select the disk and do a clean operation and then reformat the drive and it will get rid of that extra partition and create one single drive and get rid of that system recovery partition or whatever it is. I've seen this many times. Sometimes you can just right click this PC and select manage and go into the disk manager locate the drive in question, and delete all the partitions that way and reinitialize it as one volume with one partition. This is pretty common when you take a drive out of one machine and put it in another and format it, usually you have a straggling partition that is usually the recovery partition that shows up once you're done formatting it. You can also right-click it and change drive letters and things like that if you want it to be a specific drive letter other than d for instance.