Hard drive is connected through USB. We actually have a mux for the USB bus to enable three modes for the USB
1) Normal Slice activity, USB is in host mode compute module is connected to hub / ethernet controller
2) uUSB is the only thing plugged in. This brings up in device mode with the USB connected directly to the USB->SATA bridge so an external PC can read / write the disk directly
3) uUSB and DC power is connected. This brings up the device in compute module configuration mode so the uUSB is connected to the compute module, but the CM is stopped from booting normally.