It looks like you should have enough free space on your drives, but I’m not sure if the numbers given are the amount free or the amount total. You also don’t state where MongoDB data is located.
If you have 800G free on the D drive and your MongoDB data is also on the D drive, then you definitely have more than enough free space for most things. You might run into problems if you were to back up to that drive or do some other admin-y type things that would require you to basically copy the data to that drive.
If you have 800G total on the D drive and your MongoDB data is also on the D drive, then you might still have enough space for running the database. I would be concerned that I was sitting at 90% usage on the drive however, especially if your database is growing in size. Once you run out of space (as your error indicates) the system will shutdown to mitigate any potential corruption.
If you have 40 - 50G free on the C drive and your MongoDB data is also on the C drive, then again you might have enough space for running the database. I would caution again that this is not a lot of space compared to the size of the database.
If you have 40 - 50G total on the C drive then your MongoDB data will not be there because it’s just not possible. 
You can take a look at the <dbdata path>/journal/WiredTigerLog.* files to see how big in size they are on average as that might give more insight into what’s going on.