A flexible magnetic disk drive is disclosed which operates under the control of a host system. A CMOS inverter is employed as a power saving switch, holding a file protect sensor energized only when the standard "drive select" signal from the host commands operation of the disk drive. Another CMOS inverter is provided for two disk capacity a sensors which provide signals indicative of the data storage capacity of the loaded disk cartridge. The disk capacity sensors are energized only when the drive select signal commands operation of the disk drive and when the standard "motor on" signal dictates rotation of a disk drive motor.