Canon EOS-1D C Instruction Manual page 384

Troubleshooting Guide
When I shoot still photos during movie shooting, the movie
shooting stops.
To shoot still photos during movie shooting, using a CF card
compatible with UDMA transfer rates is recommended.
Setting a smaller image size for the still photos and shooting fewer
continuous shots may resolve the problem.
The time code is off.
Shooting still photos during movie shooting will cause a discrepancy
between the actual time and time code. When you want to edit a movie using
time code, it is recommended not to shoot still photos during movie shooting.
Operation Problems
I cannot change the setting with <6>, <5>, or <9>.
Set the power switch to <1> (p.38).
Check the [85: Multi function lock] setting (p.333).
Vertical-grip controls such as <6> and <9> do not work.
Set the vertical-grip operation switch to <1> (p.49).
The camera button/dial's function has changed.
Check the [85: Custom Controls] setting (p.337).
Display Problems
The file name's first character is an underscore ("_").
Set the color space to sRGB. If Adobe RGB is set, the first character
will be an underscore (p.166).
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