It is fine on my 8GB SD card and I am pretty sure that it will work up to a 32GB SD card.Good. So I'll try again today. I was on the 1.2.1 version of the firmware and I upgraded to 1.2.3 from this same 64 GB card. Anyway.
RPT, Are you loading ML for still shooting or are you into video? I just shoot stills exclusively, am interested in some of the ML capabilities that I have read about...but to be honest I have been afraid to upload it on my camera as I am not a tech-savy geek...(and a 5 D III represents a dear expenditure to me)just a very serious photographer. Hope this works for you and would love to hear how you feel about the experience if/when you get the upload to happen Mr. Big-Card! LOL! Best of LUCK!
Not to worry. I installed it via a 4GB card and it installed and uninstalled perfectly. A1ex mentioned that it now installs via a 64GB card too if the .FIR file is named in the 8.3 file name format. Will try that too.
I mostly shoot still but am interested in the raw video capability too. Looking forward to intervelometer, dual ISO and focus peaking among other things...