Canon EOS-1D X Instruction Manual page 390

Troubleshooting Guide
Flash exposure compensation cannot be set.
If flash exposure compensation has already been set with the
Speedlite, flash exposure compensation cannot be set with the
camera. When the Speedlite's flash exposure compensation is
canceled (set to 0), flash exposure compensation can be set with the
camera.
High-speed sync cannot be set in the <f> mode.
Under [z3: External Speedlite control], set [Flash sync. speed in
Av mode] to [Auto] (p.200).
The shutter makes two shooting sounds during Live View shooting.
If you use flash, the shutter will make two sounds each time you shoot
(p.208).
During Live View and movie shooting, a white
icon is displayed.
It indicates that the camera's internal temperature is high. If the white
< > icon displayed, the still photo's image quality may deteriorate. If
the red <E> icon is displayed, it indicates that the Live View or movie
shooting will soon stop automatically (p.223, 249).
Movie shooting stops by itself.
If the card's writing speed is slow, movie shooting may stop
automatically. With IPB compression, use a card with an actual
reading/writing speed of at least 10 MB/sec. With ALL-I (I-only)
compression, the card should have an actual reading/writing speed of
at least 30 MB/sec. To find out the card's speed, see the card
manufacturer's website (p.225).
If the movie shooting time reaches 29 min. 59 sec., the movie shooting
will stop automatically.
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or red E