Unable to open copied files from computer to device (mount  as disk drive) - HTC Desire S
The transfer was probably corrupted.  The problem is probably with the host OS, and not with your phone.

What OS and what program were you using to copy the files?  Did you remember to "eject" the phone from the computer to make sure everything was synchronized?

Are the files on the phone the right size?  If you plug the phone back into the computer can you look at them successfully there?  (Or try copying them off and looking at the copies.)