Messages

You got it wrong.

If (if!) v2.3 and the nightly build you intend to use do have to run on different Canon firmware version things will get complicated.
If both ML versions are compatible with the same Canon firmware version (the one you are running now) you may switch cards as you like.

ML is not firmware. It's much more like a program loaded after Windows/OS X startup.
So: If there is a nightly build running on the same Canon firmware version as you are using now you will be fine with two cards with different ML versions. If not: Much more complicated ...

Version? OS you are running? CR version? You haven't tested all the RAW editors in the wild, so please tell which one you tried.
And a DNG for downloading and testing would be fine, too.

Good for you!
If you come across the setting causing this glitch: Please report back so devs can take a look at it and - maybe - fix it for other users, too.

No, do not uninstall ML and do not format card but consult your manual:
1) Getting started -> Before you start -> Reverting the Camera to the Default Settings
13) Customizing the Camera -> [Camera] 4: Clear

Reset cam to factory defaults. Don't forget to reset C.Fn settings, too.
Reset ML settings. Shutdown cam and wait about 5 seconds after LED stopped blinking before removing battery.
Insert battery, startup cam, retry and report back. If the problem occurs try using C1-C3 mode.

http://magiclantern.fm/forum/index.php?topic=15998

If cam's firmware is close to M3 there will be not that good a chance killing anything with ML. See M3 thread -> Powershot derivate.

What the hell does "I  can get in the menu of M.L. but it doesn't start up." mean?
If you can access ML's menu: ML is running. What are we missing here?

Is there an error message? What happens when?

Accessed on bitbucket, too:
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Format card in cam.
Insert card into cardreader
Copy extracted nightly build contents to card
Insert card into cam and run firmware update.
Restart after success message and before uninstall is triggered by timeout.

Report back.

Global draw must be ON. And the overlay you want to see, of course.
If it doesn't work, link a snapshot of your Overlay tab.

Delete ML directory and Autoexec.bin. Copy extracted nightly build contents to card. Retry and report back.

My fault. There was talk about 1200x in development but never left that state.
You may contact KB support (and supportive they are!) if their replacement offer from about 2 years ago is valid still.
And make sure your cardreader does UDMA-7 or you are in risk of permanently damaging your KB card.
Here is a bunch of cardreaders tested with different cards: http://www.cameramemoryspeed.com/reviews/card-readers/
Kingston FCR-HS4 sounds quite good to me. Supports UHS-II, too.

http://www.magiclantern.fm/forum/index.php?topic=14990.msg154774#msg154774
http://chdk.setepontos.com/index.php?topic=12542.0

File cr2hdr is formerly known as cr2hdr-20bit and is the only version supported now. Same for derivates.

http://www.magiclantern.fm/forum/index.php?topic=8857.msg83814#msg83814
May I ask why you intend to buy such a card and not 1066x or 1200x? 1000x and 1050x are some sort of outdated and you may get an older stock incompatible with 5D2. And price for older/slower cards was higher the last time I checked.

Just to make sure: You have formatted them in cardreader? Try again, choose NTFS (or if on Mac: ExtFS/HFS) and insert into cam.
Cam should come up asking to format again. Do so.
For cards not working tell size/type.

Most of the time when someone writes "Same here" it is just another thing ...

Remove card, remove battery. Insert battery.
Q: Does cam startup without card?
If so: Does cam startup with card formated in cardreader?
If so: Format card in cam, make card bootable with EOSdisk/Macboot, delete card's content (don't format!) and copy extracted nightly build contents to card.

If your cam doesn't work this way please describe in detail what happens at which step and error messages/status and card used. Eye-Fi cards are considered incompatible with ML:

save them as a sort of preset?

http://www.magiclantern.fm/forum/index.php?topic=8499

Modules tab -> MLV_REC.mo
Restart
Movie tab

6D's SD-card interface is limited to about 41 MByte/s in write mode and not a single ML supported cam features UHS-II interface.

No, not together with autoexposure ...
Turn off Auto ISO in Canon menu and use Expo Lock.

Take a look into "Expo Lock" and report back, please.

There seems to be some kind of misunderstanding. Autoexposure mode is Automode (P-Mode) on steroids. You have not that much a chance to set anything on the fly in M mode. Computer takes over, Skynet is activated and no John Connor around.
You may adjust parameters, that's all.

Maybe you want to take a look into "Expo Lock" instead.