Can anyone decipher what the following is telling me? Thanks,

Jul 16 10:46:04 server6 kernel: Device sdz not ready.
Jul 16 10:46:04 server6 kernel: end_request: I/O error, dev sdz, sector 0
Jul 16 10:46:04 server6 kernel: Buffer I/O error on device sdz, logical block 0
Jul 16 10:46:05 server6 kernel: Device sdz not ready.
Jul 16 10:46:05 server6 kernel: end_request: I/O error, dev sdz, sector 0
Jul 16 10:46:05 server6 kernel: Buffer I/O error on device sdz, logical block 0