Do you have Black Magic Multibride or Black Magic Declink Extreme card?
If Declink Extreme, then they have a very serious bug which they promised to be fixed in 6.2 driver version, but it is still there in 6.2.
The bug is 2 GB RAM limit.
It means that if you want to work with FCP and BM Deck Link you must take out 2 GB of your memory and wait until they’ll fix this bug.
Good luck
Jekabs