Note that I'm assuming D is a secondary physical HDD and not just a C SSD system drive partition. Providing that's the case, then I would turn the computer back on with the D drive's cables disconnected and if everything still boots and works fine, then you could just turn the machine off and disconnect from power again, plug its SATA and power cables back in, then boot the machine back up and format it in NTFS. Be sure you back up anything you've stored on D that you may want to keep before formatting, of course.