|I assume you mean that you tried to open the files on the flash drive and that is where you had the problem.|
I also assume the files on the floppies are still OK.
If that is correct, have you tried...
Opening the files from the floppies and then *saving* them to the flash drive - as opposed to copying them?
Copying the files to your hard drive and opening them from there? If they work, save them to the flash drive.
If those methods don't help, I have used this method to find corrupt cells in Excel files:
When I need to find what is causing the corruption in a worksheet, I'll make copies of the corrupt file and then cut it in, let's say, half, to see if the corruption goes away. If not, I'll cut it in half again, and again, and again until I've narrowed it down to a single cell.
In some cases all I have had to do is re-enter a piece of data or retype a formula in the original file and the corruption has gone away.