Open a command prompt by typing CMD in the windows 10 search box.

Once the command prompt is open type in diskpart

This will open up a new window, in that window type in list volume

It should list all available volumes as seen by the disk manager.

Your will be different, but it will look something like this, your D drive will show up in there if the windows disk manager can see it. It could have been switched to hidden, or unhealthy etc... 

DISKPART> list volume

Volume ### Ltr Label Fs Type Size Status Info
---------- --- ----------- ----- ---------- ------- --------- --------
Volume 0 C OS NTFS Partition 460 GB Healthy Boot
Volume 1 ESP FAT32 Partition 150 MB Healthy System
Volume 2 WINRETOOLS NTFS Partition 990 MB Healthy Hidden
Volume 3 Image NTFS Partition 14 GB Healthy Hidden
Volume 4 DELLSUPPORT NTFS Partition 1342 MB Healthy Hidden
Volume 5 D WDC Blue NTFS Partition 931 GB Healthy
Volume 6 FAT32 Partition 100 MB Healthy Hidden
Volume 7 E Removable 0 B No Media

DISKPART>

Power saving?