Protip: If you are running linux, keep in mind that many distros will reserve 5% of your root drive as reserved (you can't see it either). This is to keep at least some space for root user if completely out of space. You can reduce this with:
tune2fs -m .5 /dev/sxx (5% -> 0.5%)
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I had a 2TB root drive wasting 100 GB as reserved space. This default should probably be changed since most drives are 500 GB or larger now.