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

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   0% /sys/fs/cgroup
/dev/md3        1.7T  1.7T     0 100% /home
/dev/md1        496M   45M  426M  10% /boot

/home indeed looks rather full.

du -hs /home suggests otherwise:

1.4G    /home

There appears no inode issue - df -i:

Filesystem        Inodes  IUsed     IFree IUse% Mounted on

/dev/md2        67108864  75334  67033530    1% /
udev             2013497    527   2012970    1% /dev
tmpfs            2015816    440   2015376    1% /run
none             2015816      2   2015814    1% /run/lock
none             2015816      1   2015815    1% /run/shm
cgroup           2015816      9   2015807    1% /sys/fs/cgroup
/dev/md3       113909760 105981 113803779    1% /home
/dev/md1          131072    239    130833    1% /boot

I recently deleted a many gigabytes of application cache and log data from /home, however this was in the tens of gigabytes at best and nowhere near the capcity of /home.

Update 1:

du -hs --apparent-size /home
1.2G    /home
du -hs /home
1.4G    /home

What might be going on here?