We can only answer questions with complete information about the issue :
Cam used , Nightly build date/version , Camera settings & the situation the camera was in when problem surfaced
FYI : (Lexar x1066) 5D2 can record 14bit 1856x1044 @23.976p +audio 44.1 in mlv_rec (full MLV) for about 2min. if not more
It's continuous @ 14bit 1856x928 23.976 +audio 44.1 (68MB/s) this all easily searchable on the forum .
3x crop 14bit continuous @ 2048x872 @23.976p + audio 44.1
Max white speed is around 79MB/s @ UDMA 6 (this maybe changeable to write @ 120MB/s with UDMA 7 with added code)