Test Unit Ready (TUR)

The SCSI Test Unit Ready command is used to determine if a device is ready to transfer data (read/write). The device will then return either good status or a check condition

SCSI communication takes place between an initiator and a target. The initiator sends a command to the target which then responds. SCSI commands are sent in a Command Descriptor Block (CDB). At the end of the command the target returns a Status Code byte which is usually 00h for success, 02h for a Check Condition (error), or 08h for busy.

When the target returns a Check Condition in response to a command, the initiator usually then issues a SCSI Request Sense command in order to obtain more information.

http://support.microsoft.com/kb/842411

To check the status of a backup device, Plug and Play sends a Test Unit Ready request to the device every second