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) SFF-8020i, MMC-2 (used by ATAPI-style CD and DVD drives) 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 devices such as USB flash drives, cameras, or digital audio players is Microsoft's FAT or FAT32 file system (with optional support for long filenames). Large, USB-based hard disks may be formatted with NTFS, which (except for Windows) is less supported. However, a keydrive or other device may be formatted with another file system (HFS Plus on an Apple Macintosh, or Ext2 on Linux, or Unix File System on 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 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 (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, 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 MB), common in solid