D: drive full, but no files found.

Microsoft is aware that previously-hidden partitions are improperly being assigned drive letters after installing the April 2018 Update. As a result, they now appear in Disk Management and File Explorer. These partitions are typically recovery partitions used by the Windows Recovery Environment or by the Original Equipment Manufacturer. It is not recommended that you attempt to delete or reformat these partitions as this may have adverse effects on your ability to recover Windows. Microsoft is currently working to address the issue in a future Windows Update, and we’ll update this thread when a solution becomes available.

(Update 5/14/18: The solution that will soon be released via Windows Update will only prevent this issue from occurring for devices that install the April 2018 Update after the solution is released. If you've already installed the April 2018 Update and are experiencing this issue, you'll need to follow the steps below if you want to hide these partitions.)

Although you can safely ignore these partitions, if you prefer do away with the low disk space warnings and hide the partitions so they no longer appear in File Explorer, you can do so by manually removing the partition’s assigned drive letter. To do this:

1. Select the Start button, type cmd and then right-click Command Prompt in the search results and select Run as administrator.
2. In the Administrator: Command Prompt window, type diskpart and press Enter.
3. Type list volume and press Enter.
4. Make note of the volume number for the drive letter you want to remove. For example, if the drive letter you want to remove is Drive “D”, note the volume number for Drive D.
5. Type the following command, and press Enter.

select volume <volume number>

When typing this command, in place of <volume number>, substitute the volume number you made note of in Step 4. In the example above, if the volume number for drive D is 5, you would type: select volume 5

6. Type the following command and press Enter.

remove letter=<drive letter>

When typing this command, in place of <drive letter>, substitute the drive letter you want to remove. In this example, you would type: remove letter=D

7. Close the Command Prompt window.

Try this fix by DaveM:

It is the recovery drive, you are not meant to see it, it is not meant to have a drive letter, remove the drive letter and everything will return to normal:

Click your Start Button, type cmd, then right click Command Prompt and choose 'Run as Administrator'

Run this command and hit Enter

diskpart

Run this command and hit Enter

select volume X

Run this command and hit Enter

remove letter=X

Close Command Prompt and the problem will be resolved . . .