I get the error "failed to create file. Card Full" when recording long takes with mlv_rec. The error happens at 40GB of recording. I have this error for the 5D3 and 7D when recording with a 64GB card or larger. g3gg0 mentioned that the problem is that there isn't a way to make more than 10 MLV files per clip.
More info here: http://www.magiclantern.fm/forum/index.php?topic=12060.0