Should you have a zero charge old main battery in there when you are testing, you can try pulling it out and charging the PRAM battery for a full 48 hours in either operating or shutdown modes, but not in sleep mode (for which the PRAM battery discharges while trying to keep memory alive while you are doing a quick main battery swap). If the PRAM loses its mind it could forget the startup disk. I didn't think this was likely based on certain Apple notes on what OS X stores in PRAM, but in mucking around with open firmware resetting NVRAM for intelligent battery resetting, I have caused that prohibitory symbol to appear...

A 20 GB drive...what partition size(s) are you using?