D: is a PC drive letter often assigned to the CD-ROM or other disc drive. Computers with one hard drive and partition will automatically assign the CD-ROM or other disc drive as the D: drive. However, this drive can be any drive letter, but is often assigned as the last physical drive letter. The picture below is an example of the DVD-RW drive, which is assigned as the E: drive.
See our drive definition for a full listing of other drives.