Scary thing happened to me. I had my FreeBSD server lock up fairly randomly. Recently got a very consistent lockup: when doing a backup. Always one particular disk (only 1 partition) caused a lockup. On the console you could still switch console screens, but you couldn't type. Also was pingable, but you couldn't ssh in. fsck never helped.
Moved all data to a different disk, rebuild the file system, and it now appears to work fine. Not nice. Must be some very weird problem.