Canon EOS-1D X Manual (en)

If you do not perform any operation while the camera and computer 
are connected, depending on the computer system, a confirmation 
message may appear on the screen asking whether you wish to 
continue communicating with the camera. If you do not perform an 
operation while this message appears, communication with your 
camera is discontinued and EU shuts down. In this case, turn the 
camera’s power switch to < OFF >, then again to < ON > and start up 
Do not put your computer to sleep mode while it is connected to the 
camera. If your computer goes into sleep mode, never disconnect the 
interface cable while your computer is still in sleep mode. Always 
wake your computer up while it is still connected to the camera. 
Depending on the type of computer, disconnecting the interface cable 
while your computer is in sleep mode could prevent the computer 
from waking up correctly. Refer to the computer User’s Manual for 
more information on sleep mode.
When a non-Canon flash is mounted on the camera for Remote Live 
View shooting, set [Silent shooting] in the camera to [Disable].
Communication between EU and the camera 
A non-Canon flash does not fire during 
Remote Live View shooting