previous a Mac formatted system? haha I don't understand this. I just bought the HD from Challenger then plug in straight use, never format or anything before if that is what u mean.
well forget the norm of technical issues, as in now currently ur hdd is unreadable and data extraction might need a software to do it, but im not sure if most software can read drive in unallocated drive or not, beside that im not even sure your all your data still exist or only some still exist.
but if your hdd really have no data u can use girmgh method.
next i wont reco this method if your ext hdd have seal on the screw holes or around the case, but if u can, dismantle your your ext hdd and plug the hdd to your pc and see if you can read any data or show the same results. cos im suspecting might be the ext hdd controller have issues.