Running out of disk space on a SQL Server server is something that you should never allow to happen, but it can happen.
If you need to be the emergency Fireman putting out the fire, here is a way to “reserve” some drive space ahead of time.
It is literally a dummy file that you can delete at any time without worry – nothing is using the file.
fsutil.exe file createnew "Emergency Disk Space (safe to delete).$$$" 8589934592
This will create an 8 GB temporary file in the current folder. Adjust the size to suit your needs.
When you need to reclaim the disk space, simply select the file and shift+delete (no need to send it to the Recycle Bin).
Here’s the gist if you don’t file like copy&pasting.