Hi Alexander, this is a known issue in the 1803 Update,
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
list volume
Note down the letter associated with that new drive
Run these commands one at a time and hit Enter (replace X with the correct drive letter)
select volume X
remove letter=X
Close Command Prompt and restart your PC