After the 1803 update, some PCs seem to create a D drive. The data in that drive is used during the upgrade process and is suppose to be removed once the update is completed. Although now it's completed the data is actually no longer required.
I have found that simply right clicking the drive, and selecting "Format..." is sufficient to stop the low disk space warnings.
If you really want to nuke it; you'll need to first format it, then use diskpart to unassign the partition and set its attribute to hide.
- Open an elevated command prompt, type the diskpart commands and press Enter.
- Press the windows key and type cmd. Right click Command Prompt and select Run as Administrator.
- Type the command list volume
- Note the number of your D drive.
- Type the command select volume <volume number>, for example select volume 5
- Type the command remove letter="<drive letter>, for example remove letter="D
Disk part will remove the drive letter.
Hopefully this is helpful to someone!