Does you external hard drive have recovery feature? If it does, and you have backup, you can use this function to recover lost data. From your description, I think your hard drive get bad sector. Anyway, if your hard drive can't recover data by itself, you can use recovery program to recover data from external hard drive, know more here: External Hard Drive data recovery.
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Take the drive out of its enclosure and connect it directly to the PC using an adapter kit if needed. If it still doesn't function, then tools like Get Data Back, and Linux boot disk with testdisk on it are the order of the day. The Linux solution is for pro's though. You can easily wipe everything.
There are pay-for services that can often recover data on hard drives which would otherwise be little more than scrap. They can be found by searching for "data recovery services" in google and related search engines.. AFAIK, they are not cheap.