CF to SD adapter - 5DII works, 7D plays dead

[Intenditore]

It's not quite an ML-related topic, but I don't know a community more tech-savvy to ask it. So I'll be glad if you can help.
I've got a CF to SD adapter which works nicely with 5DII. It's working the same way in card reader. But as I plug it into my 7D I bought it for it doesn't start/boot - no signs of life at all. After I remove it - everything is fine.
It works normally with Lexar CF though.
I also noticed one interesting thing. I have an old "bad" battery giving "Battery communication error. Use this battery?". It works though.
And if I plug the adapter and try to start the camera, than remove this battery and after some time plug it in again - I get a blinking battery on the top lcd 
Nothing else happens though.
Can you give me a clue, what is this and how to get it working?

[Walter Schulz]

Take a standard CF card. Copy nightly build for 7D on it and run installation. Wait for 60 seconds for timeout to deinstall camera bootflag.
Now test again with adaptered card.

[Intenditore]

Wuaauw 
It worked!
I thought bootflag is an attribute of a card, not camera, so didn't even think previews ML installation could interfere with it somehow. Especially if it works with another card.
Many thanks, that was easy)

[Walter Schulz]

https://wiki.magiclantern.fm/glossary