gate:~ dmesg
...
[ 4220.798665] ide: failed opcode was: unknown
[ 4220.798665] end_request: I/O error, dev hda, sector 21067462
[ 4222.983683] hda: dma_intr: status=0x51 { DriveReady SeekComplete Error }
[ 4222.983683] hda: dma_intr: error=0x40 { UncorrectableError }, LBAsect=21067572, sector=21067470
...
Lucky for me there are two disks in RAID 1 so my data was not lost. The machine is "just" firewall, so I decided to play little bit with the bad hard disk, because there are no important data on it. Usually if you see errors like I mentioned above you replace disk without any questions, but I would like to "get" some outputs from diagnostic commands. So you can see what you can do in such case.