Go to Notifications & actions in settings. The easiest way to do this is by typing Notifications in Cortana search. You could also get to the Notifications & actions screen by pressing Windows Key + I -> System -> Notifications & actions .
Scroll down the Notification & actions screen and click on Windows Explorer.
Choose to turn off notifications or change the settings for Windows Explorer to suit your needs.
This will turn off the notification for low hard disk space, however this will also turn off all notifications from Windows explorer.
At the moment there is no settings to turn off the notification for a single drive. If you have a solution to turn off the notification for a single drive, commodo permissum nos teneo in ineo inferius.