BulkOnly

BulkOnly, short for Bulk-Only Transport (BOT), is the transport protocol used by most USB Mass Storage Class devices to carry SCSI commands over USB. Defined by the USB Implementers Forum as part of the USB mass storage specification, BOT relies on the device’s bulk endpoints rather than any dedicated control channel. It is designed to be simple and broadly compatible, which contributed to its wide adoption on USB flash drives, external hard disks, and similar storage devices.

In BOT, the host and device communicate through two bulk endpoints: bulk-out for command transmission and bulk-in for data and status. The host first sends a Command Block Wrapper (CBW), a 31-byte structure that contains a signature, a command tag, the data transfer length, a direction flag, the length of the command descriptor block (CDB), and the SCSI CDB itself. The device then performs the command and, if data transfer is required, moves data over the appropriate bulk endpoint. Afterward, the device returns a Command Status Wrapper (CSW), a 13-byte structure indicating the outcome and including a tag that matches the CBW.

The CDB within BOT is a SCSI command descriptor block, so common operations include INQUIRY, READ CAPACITY, READ, and WRITE. The CSW reports status as good, failed, or a phase error; error details may be provided via SCSI sense data in the data phase.

BOT remains widely supported for compatibility, though USB Attached SCSI (UAS) offers improved performance and error handling on newer USB interfaces. See also USB Mass Storage Class, SCSI, and UAS.