Yeah man .... thats crazy. I've built many computers and changed several PSU and never once have I gotten an error from simply swapping out a PSU.
I'd stick the old PSU back in ... if you no longer get the boot error, I'd take the Antec back. As it's always possible to get a bad unit. Though Antec is very reputable, I guess it's possible they screw up too.
I can see how adding the new drive could cause problems with the power supply, as it is requiring more demand from the PSU to run that one more device. Seeing as you always had two drives, my only guess is the new drive you got draws just enough more current than the old one to put a strain on the 300 watt PSU.