Copying from CD problems

I recently compressed some folders with 7zip, and parted it into two parts. I copied one part onto one disc and the other part to another disc. Now, when I try and drag and drop the first half, it comes up with an error like, invalid MSdos command (or something). When I try to open it with 7zip and extract it onto the desktop, it comes up with another error. There was over 2500 of my work files and folders, so I really need to fix this. Thanks a million.