OP:
When you boot (with the external drive attached), are you holding down the option key to invoke the startup manager?
Final thought...
At approaching 15 years old, I wouldn't spend much (any?) money on this.
A bootable USB flashdrive may work. But then again, it may not.
Sometimes it's best to say "thanks for the memories"... then replace the old Mac with something new (or late-model used) and take it to recycling...