Editing USB mass storage device class (section)

==Device access==
[[File:Usbcardreader.png|thumb|alt=A small, thin, gray box, with data card inserted in a bottom slot|USB [[card reader]]s typically implement the USB mass storage device class.]]
{{see also|USB Attached SCSI}}
{{missing information|section|Bulk-Only Transport: we keep linking to this article to contrast with UAS but there's no description here!|date=May 2023}}
The USB mass-storage specification provides an interface to a number of industry-standard command sets, allowing a device to disclose its subclass. In practice, there is little support for specifying a command set via its subclass; most drivers only support the [[SCSI command|SCSI transparent command set]], designating their subset of the SCSI command set with their SCSI Peripheral Device Type (PDT). Subclass codes specify the following command sets:
# Reduced Block Commands (RBC)
# [[Small Form Factor committee|SFF]]-8020i, [[MultiMedia Commands|MMC]]-2 (used by ATAPI-style CD and DVD drives)
# [[Quarter-inch cartridge|QIC]]-157 (tape drives)
# [[Uniform Floppy Interface]] (UFI)
# SFF-8070i (used by ARMD-style devices)
# SCSI transparent command set (use "inquiry" to obtain the PDT)

The specification does not require a particular [[file system]] on conforming devices. Based on the specified command set and any subset, it provides a means to read and write sectors of data (similar to the low-level interface used to access a [[hard drive]]). Operating systems may treat a USB mass-storage device like a hard drive; users may partition it in any format (such as MBR and GPT), and format it with any file system.

Because of its relative simplicity, the most common file system on [[embedded system|embedded devices]] such as [[USB flash drive]]s, cameras, or digital audio players is Microsoft's [[File Allocation Table|FAT]] or [[FAT32]] file system (with optional support for [[long filename]]s). However, USB mass storage devices may be formatted with any other file system, such as [[NTFS]] on Windows NT, [[HFS Plus]] on [[macOS]], [[ext2]] on [[Linux]], or [[Unix File System]] on [[Solaris (operating system)|Solaris]] or BSD. This choice may limit (or prevent) access to a device's contents by equipment using a different operating system. OS-dependent storage options include [[Logical volume management|LVM]], partition tables and software encryption.

In cameras, MP3 players and similar devices which must access a file system independent of an external host, the FAT32 file system is preferred by manufacturers. All such devices halt their file-system ([[umount|dismount]]) before making it available to a host operating system to prevent file-system corruption or other damage (although it is theoretically possible for both devices to use read-only mode or a [[cluster file system]]). Some devices have a write-protection switch (or option) allowing them to be used in read-only mode.

Two main partitioning schemes are used by vendors of pre-formatted devices. One puts the file system (usually FAT32) directly on the device without partitioning, making it start from sector 0 without additional boot sectors, [[Header (computing)|headers]] or partitions. The other uses a DOS partition table (and MBR code), with one partition spanning the entire device. This partition is often aligned to a high power of two of the sectors (such as 1 or 2&nbsp;MB), common in [[SSD|solid state drives]] for performance and durability. Some devices with embedded storage resembling a USB mass-storage device (such as MP3 players with a USB port) will report a damaged (or missing) file system if they are reformatted with a different file system. However, most default-partition devices may be repartitioned (by reducing the first partition and file system) with additional partitions. Such devices will use the first partition for their own operations; after connecting to the host system, all partitions are available.

Devices connected by a single USB port may function as multiple USB devices, one of which is a USB mass-storage device. This simplifies distribution and access to drivers and documentation, primarily for the Microsoft Windows and Mac OS X operating systems. Such drivers are required to make full use of the device, usually because it does not fit a standard USB class or has additional functionality. An embedded USB mass-storage device makes it possible to install additional drivers without CD-ROM disks, floppies or Internet access to a vendor website; this is important, since many modern systems are supplied without optical or floppy drives. Internet access may be unavailable because the device provides network access (wireless, [[GSM]] or Ethernet cards). The embedded USB mass storage is usually made permanently read-only by the vendor, preventing accidental corruption and use for other purposes (although it may be updated with proprietary protocols when performing a firmware upgrade). Advantages of this method of distribution are lower cost, simplified installation and ensuring driver portability.