A XFS file system can become corrupted due to a power cut, a kernel bug or something else. To repair it, please use a recent version of xfs_repair (Debian package: xfsprogs) (I would recommend at least version 2.9.8).
umount partition (umount /dev/sdb1)
xfs_repair it (xfs_repair /dev/sdb1)
This might take a while and in the end should succeed, lost data is then put under lost+found under the root of the file system.
-- CarstenAulbert - 10 Feb 2009