Files bad after transferring to external hard drive

After transferring all of my saved files (pictures, videos, mp3s) from my 500gb Seagate external hard drive to my new Western Digital 2tb hard drive, over half of everything has been corrupted. I don't know why, I am devastated right now.

When I first started transferring all of my files to my new external hard drive, I realized I had to format it in order to transfer large files. So I had to go back and copy everything right back to my old hard drive. After I finished formatting the new one, I copied everything back to it. Now not only are most of my files from the new one is screwed up, but most everything on my old external hard drive is corrupt too. Is there a way to fix the files that has been damaged and why did this happen in the first place?

screwed up how? can you be specific? If the files were corrupted on the old hard drive and you attempted to copy them the OS would give you errors while copying them. If you did not get an error message while copying them than the OS (I assume windows) did not see them as corrupted.

For example, some of the avi movie files would play then suddenly stop. Some just won't play all together. Most of my pictures are missing pieces to them or have just vanished. Pdf files are screwed up to where pages are switched around.

That is not hard drive corruption. If the files were really corrupted at the system level you would not even be able to open them up, you would get an error. Have you checked for malware?

I ran a full system scan and found nothing. Is there a way to repair my files? I'd just like to know why this happened in the first place. I was just transferring everything to a new external hard drive from my old external hard drive.

If the files are screwed up on both hard drives then I dont think the transfer/copy caused the problem. How did you scan the system?