Just posting this here in case anyone else runs across this error message, or one like it:
"...unable to read config index 0..."
My Arch Linux installation had been working perfectly up until this point
This message comes up right after the multi-color screen while booting Arch Linux, directly after the "RTC not found" message.
I spent quite a bit of time researching non-RPi forums (because there was nothing about this error on the RPi forum), and still didn't get the problem resolved.
Basically, any device plugged into the USB ports would trigger this message (and a bunch of related errors), even a plain USB keyboard. However, Arch Linux would boot all the way to the login prompt at which point I was stuck, because I couldn't use a keyboard to login.
I had been using the exact same power brick (5V 850mA) that I had been using before I got this error message. The only difference: A different USB cable. I found the USB cable I had been using and switched it back, and voila, it all started working again.
So, hopefully this will save you some time if you have the same or similar error message!