I'm posting this in case it happens to others and also as a searchable resource for myself if I forget it again!

I've experienced a number of system lockups with the 1D but I think it may be applicable to the 7D and 5D also. LCD reads "Error 70" and freezes the system. It has taken a while to develop a methodology that seems to clear it up reliably. It's pretty convoluted and entirely possible that it may be possible to skip a few steps, but because the error code shows up without any rhyme or reason (and always at the worst possible time), I never have the luxury of trial and error to work out variations.

I'm always using the Blackmagic HDMI converter with my camera, so what I do is this:

--power down camera and remove battery
--power down Blackmagic
--unplug the HDMI from the body, then plug it back in
--power the Blackmagic
--insert battery and power up camera

Why this works I have no idea, perhaps it's something in the HDMI handshake. All I know is, it seems to work.

I love you Charles.
I was on the verge of sending in my T2i which was completely non functional due to Error 70.

But i decided to search the web and after probably an hour found your post.

In my case it was a Lilliput HDMI monitor but I went through your steps and bingo its fixed. Must have had something to do with the handshake when I previously used my HDMI monitor.

Have you had it happen again and figured out the cause?

Lenny

Awww, I love you too Lenny.

OK, now that we've gotten that out of the way...!

I think you right, the camera seems to get stuck in a handshake loop that it can't get out of and freezes. Honestly I can't say. All I know is--glad I know how to get out of it now. It has happened to me a few times since. Nothing I can trace back to any specific action. Just business as usual.

Glad I was able to help!