It seems that anything that was on the external HD is gone beyond recall by any "normal" means
You haven't said what make of HD you have, but Maxtor for one do have a download from their website for most of their HDs. This makes short work of formatting and partitioning and though I haven't tried it with other makes, I think you may be able to use "Maxblast3" as it calls itself.
Probably other makes have equivalent downloads available.
At best this will only give you access to your new HD, any data on it before has gone.
If you can find a suitable download, then load the USB software first and then when rebooted plug in or switch on the HD and see what happens.
If the download is like MaxBlast3 then you will have to amke a floppy and reboot with it in the slot.
All this is just my guess, but I think I know how you feel right now.