Cannot get SD card to work

Ok, I've done quite a lot of checking and fiddling now with a signal analyser, and:

• Some cards seemed to work reliably as-is
  
• Some cards worked reliably if you tried to initialise them a second time after the first time had failed
  
• Some cards wouldn't work at all *unless you added a pullup resistor to the CS/CD pin)
  
  

So I have modified the firmware now so that it automatically retries the initialisation a few times, which fixes a lot of problems... And I'll modify the documentation to suggest adding a pullup resistor to CS (which is good practice anyway as it'll stop noise from inadvertantly writing rubbish to the card, even if it's amazingly unlikely).