It's standard advice to always have a good backup of all critical files.. If you've got a 400 GB drive full of important files, you should have a second 400 GB drive to use as a backup. (Of course it's also possible to make another reliable method of making copies of the files such as DVDs, etc.).. Do you have such a backup?

If no such backup exists, then you'll need to stop performing any work on the removable HD and decide how important the files are.. For marginally important files, there are free recovery software options or if the data is extremely important, try a professional.

Hope this helps.