Some people notice after upgrading to Windows 10 April Update, that there is a new partition (recovery partition with Windows PE). Due to the fact, that a drive letter is assigned, the logical drive will be shown in explorer. And Windows 10 starts with annoying notifications, that the disk is full. This forum post describes the situation:

Disk management is failing

The first idea many users have is to remove the drive letter from the disk management partition. If you want to use Disk Management to fix the issue and remove the drive letter from the partition, you will fail in most cases. The context menu does not support commands on this recovery partition.

Open the command prompt window with administrator privileges

diskpart

list volume

select volume <number of your volume>

remove letter=<drive letter of your volume>

exit

( not able to remove letter from drive windows 10 1803 )

Windows 10 V1803 update creates a new OEM Partition