A Memory Technology Device (MTD) is a type of device file in Linux for interacting with flash memory. The MTD subsystem was created to provide an abstraction layer between the hardware-specific device drivers and higher-level applications. Although character and block device files already existed, their semantics don't map well to the way that flash memory devices operate. USB sticks, MMCs, SDs, CompactFlashes and other popular removable devices are not MTDs. Although they contain flash memory, this is hidden behind a block device interface using a Flash Translation Layer.