Mhm... too happy, too soon. It DID work, for a while; as a quick test I recorded a 15 minutes , 1.65 GB video. But then I popped the card into the computer, checked the video (it was fine), deleted it...
... and now the camera won't do anything with that card. "Memory card error" is all it says. Reformatting and reusing SDMInst doesn't change anything. What can have changed?? I have already tried retracing my steps from the blank-from-the-shop card to the successful CHDK boot. No dice.
Bummer. I guess I'll better start posting in the questions section.
Regards.
