What are Sense Codes?

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Sense data contains detailed information about error conditions. It is organized into major categories called sense keys and sub categories called additional sense codes (ASC) and additional sense code qualifiers (ASCQ). The combination of these data fields can finely convey detailed information about the error condition.

Whenever a command is sent to a SCSI device, the sense data is made available to the device driver.

The sense keys are generic, and have the same meaning regardless of the type of device. For example, sense key #1 means to "Check Condition", but the command completed. S.M.A.R.T. Disk Monitor queries the additional sense code and might report that "Fly Height Change Problem, Recommend Device Replacement". Which would be the case if you had an IBM DGHS Ultrastar, and received sense Code of 5d, qualifier 00, additional codes 02 25.