deathromantik, I have encountered some success with restoring hard drives to useful service. One tool of particular good use is the 'HDD Rejuvenator' program found on the 'Hiren's Boot CD' series; I have found it to be reliable and effective, in recovering sectors and restoring drives to full service. The various manufacturer's utilities are present as well, and may at times be of some use also. Just follow the instructions that appear on-screen as you use these utilities, and you'll likely find them to be mostly self-explanatory.
I've not yet attempted such with an external USB drive, however. The Hiren's disc is pretty much DOS-based, and I'm not certain at all of however well its DOS-mode USB driver(s) may or may not handle such drives. Yet I've encountered no difficulty at all with the more 'traditional' setups, with the drive directly connected - and since that method is reliable, you could also choose to perhaps remove the hard disk drive from its plastic USB shell and connect it directly too. With the bonus being the additional knowledge of how to remove the drive from the shell successfully, which may later be handy for installing a new replacement drive unit, as well. (Just in case the original drive does not recover successfully.... ;-)