I had a need today to test a disk space threshold alert and needed to fill up my hard drive. I started by copying files over and over, but that was taking a long time. A little searching and I found fsutil.exe
fsutil file createnew <filename> <bytes>
This quick little command will create a file of the specified size. Just view the hard disk properties for the amount of free space (which it kindly provides in bytes), subtract a few to keep from maxing it out, and run fsutil.