Packet command error on hard disk

Please, can somebody explain the following entries in the error log? There are hundreds of them. Does it mean a bad disk drive? This occurs infrequently (once every few days) and goes for about an hour, then suddenly goes away. Nov 8 16:00:02 tux kernel: hda: packet command error: error=0x54 Nov 8 16:00:02 tux kernel: ide: failed opcode was 100 Nov 8 16:00:03 tux kernel: hda: packet command error: status=0x51 { DriveReadySeekComplete Error } Thanks Peter