XFS disk half full, yet it is reporting out of disk (linux)

[Krzysztof Adamski <k@xxxxxxxxxxx>]

I'm running a 64bit kernel Linux (Debian Lenny) self compiled kernel
2.6.30.4. I have created a 5T xfs filesystem, it is about half full but
I get "No space left on device"

Here is the output of df:
gen:/fstore-1# df .
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/mega--raid--1-fstore--1
                     5265707008 2711698716 2554008292  52% /fstore-1
gen:/fstore-1# df -i .
Filesystem            Inodes   IUsed   IFree IUse% Mounted on
/dev/mapper/mega--raid--1-fstore--1
                     1053167616  736704 1052430912    1% /fstore-1
gen:/fstore-1# >abc
-bash: abc: No space left on device

What limit did I hit?

K