How to recover data from external hard drive

Since the drive is 320GB I would assume its using NTFS. I would highly recommend Recuva in this case, and if that fails, TestDisk can do a more thorough job.

Try Recuva from Piriform:

Accidentally deleted an important file? Lost something important when your computer crashed? No problem! Recuva recovers files deleted from your Windows computer, Recycle Bin, digital camera card, or MP3 player. And it's free!

Regardless of wehre this question should be posted...

Don't do anything else... Just because you can't see the files, doens't mean that they're gone.

If your using a Windows PC, use PC Inspector File Recovery