Hi
There are a number of programs which can undelete a hdd internal or external
Providing you have not written anything to the external hdd since the problem most files will be recovered
You have to recover to a different hdd than the one they came from
I would try Piriform recuva and sgsecurity photorec(overy)
Other programs by http://www.easeus.com/
See which does a better job of recovering folder & file names
Once you have ensurred you have a full recovery of your files you can copy files back to the external hdd
(If very valuable data consider professional data recovery service)
Regards
Mike Barnes