Filesystem full? I've got 4.2TB free!

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

Filesystem in question is:

Filesystem  Size Used      Avail %Cap    iUsed   iAvail %iCap Mounted on
/dev/dk0    4.4T  39G       4.2T   0%   224044 592051282   0% /backup

netmanager 1# ls -l /backup
total 14
drwxr-xr-x  3 root  wheel  512 Apr  6 11:38 Mon
drwxr-xr-x  3 root  wheel  512 Apr  6 11:54 Mon-1
drwxr-xr-x  3 root  wheel  512 Apr 21 17:26 Tue
drwxr-xr-x  2 root  wheel  512 Apr 22 00:25 Tue-1
drwxr-xr-x  4 root  wheel  512 Apr  1 19:11 apollo
drwxr-xr-x  3 root  wheel  512 Apr 21 23:00 dellboy
-rw-r--r--  1 root  wheel   81 Apr 22 00:32 lastrun.txt
netmanager 2# mkdir /backup/test
netmanager 3# mkdir /backup/dellboy/test

/backup: write failed, file system is full
mkdir: /backup/dellboy/test: No space left on device

This is on 5.0_RC2. I've tried with and without WAPBL and also fscking 
(the machine did have a crash yesterday).

Any clue to the phantom filesystem full?

--
Stephen