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Mass Storage FAQ

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Mass storage page


How long can a device return NAK in a mass-storage bulk transfer?

The mass-storage specification doesn't define a timeout value. Hosts typically will wait 20-30 seconds.

Can my device's firmware read and store files when the device isn't attached to a host?

Yes, if the firmware supports a file system such as FAT16 or FAT32. Note that the mass-storage device should have only one mass-storage master at a time. When the device is attached to and configured by a USB host, the firmware shouldn't attempt to read or write to files on its own.


What is the SCSI transparent command set?

In the Mass Storage Class Specification Overview, Table 2.1 says that devices with subclass code 06h implement the "SCSI transparent command set." The SCSI specifications are available from, but these documents don't mention a transparent command set.

For subclass 06h, the host should determine the device type by issuing a SCSI INQUIRY command. In the returned INQUIRY data, bits 4..0 of byte 0 specify a peripheral device type (PDT). The SCSI Primary Commands (SPC) specification defines various PDTs and the specifications they should comply with.

What value should I use for bInterfaceSubClass in the interface descriptor?

New designs should use bInterfaceSubClass = 06h (SCSI transparent command set). The device's response to a SCSI INQUIRY command is then the single source where the device declares its command set (via the PERIPHERAL DEVICE TYPE (PDT) and VERSION codes). A device should use a different bInterfaceSubClass only if it must for some reason adhere to a command set that has no PDT code (such as SFF-8070i). See the SPC document for a list of PDTs.

What commands should my device support?

In the response to a SCSI INQUIRY command, a device returns a PERIPHERAL DEVICE TYPE code and a VERSION code. For hard drives, flash drives, and similar devices, PERIPHERAL DEVICE TYPE = 00h (direct access block device) and VERSION = 04h (SPC-2) or 05h (SPC-3). The code in the VERSION corresponds to a command set, and the command set's specification lists mandatory commands.

In some cases, additional INQUIRY data can provide more information, including vendor-specific data, but not every host will retrieve more than the first 36 bytes of the response.

The SPC specification documents the INQUIRY command. Also see the specification for your device type for any device-specific information about the INQUIRY response.

In practice, many devices don't implement every mandatory command and just concentrate on the commands used by any hosts the device is likely to connect to.

The Mass Storage Class Compliance Test Specification names required and optional commands for different PDTs that want to pass USB-IF's mass-storage compliance tests.

What is command 0x23 (23h)? I can't find it in the SPC or SBC documents.

Command 23h is READ FORMAT CAPACITIES in the Multimedia Commands (MMC) specification from

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Windows Specific

Can my device use the Reduced Block Command (RBC) set?

Windows doesn't support RBC (bInterfaceSubClass 01h or 06h and PDT = 0Eh), so if you design an RBC device, you'll need to provide a driver for the host. The USB mass storage overview document from incorrectly says that flash drives typically use RBC. Flash drives use bInterfaceSubClass 06h (SCSI transparent command set) with PDT = 00h (direct-access block device), the same as hard drives.

How can I determine the drive letter Windows has assigned to a USB drive?

Several options:

Example code from Marc Reinig.

Usenet thread How to match between physical usb device and its drive letter.

Using WMI