Problem copying files to USB drive, the files get altered/corrupt.

I'm having problems when I copy files from the internal drive to any one of my external USB drives, it appears that the files are altered (or gets corrupt) in some way while copying. All drives are formatted using Mac OS Extended (journaled and case sensitive), I've tested them and there are no bad sectors or any other errors on the drives themself.

I mostly use the drives for backing up movie files and I started noticing a few weeks ago that many of the recent copied movies on the USB drive had a lot bad frames and other problems when I played them. I checked the original files on the internal hard drive and played them and the playback went flawless.

I've compared the md5 checksum of both files using the terminal and hash doesn't match, so I guess the file on the USB drive isn't an exact copy of the original?

I just recently started to copy files from the internal drive to the external USB drives using Finder. Earlier I usually copied the files to the USB drives through my network using a FTP server and Cyberduck. The files copied using FTP appears to be correctly copied without any errors so I conclude that it's the copying in OS X and the FInder that creates to problems.

Does any one have any suggestions what I could test or what could be wrong? Obviously OS X and Finder should be able to copy files without any errors, is there "safe copy" mode I can switch on or anything when copying files using the finder?