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.

 

https://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