Ubuntu "No space left on device" for /home, df shows 100% full, ds shows much, much less

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http://serverfault.com – On an Ubuntu 12.04 server, normal users can no longer create or add to files in /home, encountering a "No space left on device" error. The /home directory has a capacity of 1.7 terabytes and as far as I can tell is nowhere near full in terms of actual data stored or inodes used. df -h shows: Filesystem Size Used Avail Use% Mounted on /dev/md2 1.0T 18G 955G 2% / udev 7.7G 4.0K 7.7G 1% /dev tmpfs 3.1G 320K 3.1G 1% /run none 5.0M 0 5.0M 0% /run/lock none 7.7G 0 7.7G 0% /run/shm cgroup 7.7G 0 7.7G (HowTos)