It's an old post, but have almost the same problem.
The system hangs with code 51 and doesn't boot further, when the cardreader is attached and even an USB stick give the problem.
What i noticed is that with an USB Stick: The activity LED of the USB stick is blinking constantly.
Also another strange thing is that it happens like 5 of the 10 Boots runs for example. The other 5 boot runs i have no problems.
BIOS is up to date (013). Any known solutions for it or does have more people the same problem?
Message was edited by: v.kamphuis
Ok after days of trying.
I've replaced the motherboard and it looks like that solved the issue.
Also after a few days I had some other problems. Seems like 1 of the memory modules was faulty.
But the issue above appears to be a faulty motherboard, after changing it the issue was solved.
Will test the motherboard with some good memory. If the problem remains, it's for sure, The motherboard was causing it.
Otherwise I'll post it here.