I have a storage drive (2TB) and an OS drive (90GB SSD). I've run out of space on the OS drive:

/$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb1        72G   72G     0 100% /
udev            5.9G   12K  5.9G   1% /dev
tmpfs           2.4G  1.2M  2.4G   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            5.9G  428K  5.9G   1% /run/shm
/dev/sda1       1.9T  639G  1.2T  37% /media/StorageDrive

So be it. But when I attempt to figure out where the space has gone, I cannot find it anything remotely approaching the capacity of the drive:

/$ sudo du -h -d 1
du: cannot access `./media/StorageDrive/home/ari/.gvfs': Permission denied
675G    ./media
2.3G    ./var
0   ./proc
7.0M    ./tmp
27M ./boot
4.0K    ./lib64
12K ./dev
44M ./home
16K ./lost+found
8.0M    ./sbin
223M    ./lib
4.0K    ./selinux
1.4M    ./run
140K    ./root
8.8M    ./bin
4.0K    ./mnt
38M ./etc
8.0K    ./srv
4.8G    ./usr
65M ./opt
0   ./sys
682G    .

Note the difference between the total (682G) and the mounted drives in /media (675G) is only about 9G. How are 72G being used? Where is this dark matter hiding?