On a disk, a media error is an event where the disk was unable to perform the requested I/O operation because of problems accessing the stored data. Media errors are classified into recoverable and unrecoverable.
Recoverable errors are more common and so have a higher expected error rate. Recoverable errors indicate that a disk may have to do something more than the initial access to correctly access the data. The disk was ultimately able to provide the requested data with no intervention by RAID. It may be that that a retry is needed, or that the retries do not produce the correct data and that some error correction has been done.
Recoverable media error:
[ispfc_main:error]: Disk 8a.22: op 0x28:0002c118:0080 sector 180549 recovered error - (1 17, 3)
[scsi.cmd.checkCondition:error]: Device 8a.22: Check Condition: CDB 0x28:0002c118:0080: Sense Data recovered error - (0x1 - 0x17 0x3 0xe4).
Unrecoverable media error:
[ispfc_main:error]: Disk 7a.38: op 0x28:023d3540:0080 sector 37565872 medium error - Unrecovered read error (3 11, 0)
[ispfc_main:notice]: Disk 7a.38: sector 37565872 will be reassigned
[ispfc_main:error]: Medium err for disk 7a.38 (serial no. 12345678000072011ACT).
[scsi.cmd.checkCondition:error]: Device 7a.38: Check Condition: CDB 0x28:023d3540:0080: Sense Data medium error - Unrecovered read error (0x3 - 0x11 0x0 0xe4).
More information can be found in Understanding Media and Recovered Disk Errors.