Canon EOS 5Ds Instruction Manual page 481

When I shoot still photos during movie shooting,
the movie shooting stops.
To shoot still photos during movie shooting, using a CF card capable
of UDMA transfer or a UHS-I SD card is recommended.
Setting a lower image quality for still photos or shooting fewer
continuous still photos may resolve the problem.
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 the <6>, <5> or <9>.
Set the <R> switch to the left (lock release, p.59).
Check the [83: Multi function lock] setting (p.410).
A camera button or dial does not work as expected.
Check the [83: Custom Controls] setting (p.413).
Troubleshooting Guide
481