I got the some issue (ERR 70 when updating with a 5D mark iii) and solved it with the same card on a 5Ds: strange enough as I never got any trouble in updating lens or reflex firmwaresThe update gives me ERR 70 trying to update with a 5D-IV :-( I tried with two different SD SanDisk cards formatted in camera (SDXC II Extreme Pro 64GB and SDXC I Extreme Pro 512GB).
I had to reset the camera putting out the battery to recover the camera functions.
SOLVED 7 nov: I tried again with an old SD card (SanDisk Extreme Pro SD HC I 16 GB) and the firmware update worked flawlessly.
I just updated the 400mm DO II using the adapter on the R5, which is the same process - it was simple and fast. I haven’t done it on the 100-400mm II yet as no problems with it after 5000 shots.Anyone update the firmware using an R5 / RP / R6 with an adapter? I have not yet given this lens a workout on the RF -Adapter mount but it is a great lens that will see use for a while. I am not ready to get the RF 100-500 yet.