media errors - SCSI disk

Media errors mean physical defects on the disk platter surfaces (simplified explaination).

chkdsk will check for and mark off physical media errors, but has limitations, such as having to unmount the volume involved to have physical errors repaired, and the length of time it takes.

vlaurie.com/.../chkdsk.htm

Most Dell raid controllers in use have a firmware based util call Patrol Reads, which as a default runs automatically. It checks for physical errors on the entire disk surface of hard disks in arrays. Consistency checks only check the consistency of the area on the platters used for data (parity and physical media), which is important, but patrol reads is more important. Insure patrol reads is enabled... many times an array will fail during a rebuild due to errors on physical areas of arrays not containing data, as this area is not normally checked for errors. Aside from this, physical areas of the disk array containing data are checked automatically by the controller as the raid adapter reads info on a continuous basis. Logs are created for both consistency checks and patrol reads, as to how long a CC takes, depends on how much data resides on the array.

You generally start the consistancy check and then go a head and boot and let the production use continue while it's doing the consistancy checks.