SCSI Commands - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 135

On receiving an unsupported command, a device must not crash or hang.
The correct response to an unsupported command is this:
• Return 01h (command failed) in the CSW 's bCSWStatus field.

• In the sense data, set the SENSE KEY parameter to 05h (ILLEGAL

REQUEST) and set the ADDITIONAL SENSE CODE parameter to
20h (INVALID COMMAND OPERATION CODE), as described
below.

Sense Data
On experiencing a problem in executing a command or on receiving an
unsupported command, a device fills a structure with status information
and sets bCSWStatus in the CSW to 01h (command failed). The status
information is called the sense data. A REQUEST SENSE command can
request the sense data, which the device returns in the data-transport phase.
A device can also use sense data to announce other events that require atten-
tion by the host, such as the inserting of removable media. To signal the
event, the device sets the SENSE KEY field in the sense data to 06h (UNIT
ATTENTION).
The contents of the returned sense-data structure vary with the command,
the device type, and whether the DESC bit in the REQUEST SENSE com-
mand block requests fixed-format sense data (0) or descriptor-format sense
data (1). Fixed-format sense data uses a single defined structure to return
status information (Table 6-2). The format of descriptor-format sense data
varies with the descriptors being sent. Each descriptor is a structure with a
type of status information such as command-specific information, informa-
tion relating to an exception condition, or information relating to block
commands.
(In true SCSI communications, the device returns a status code after each
command, and a status of CHECK CONDITION indicates that the SCSI
host should issue a REQUEST SENSE command. USB communications
use the status code in the CSW instead.)