BOAC, I had a similar thing once and it was to do with the copy protection on the disks, can't remember the exact why's and wherefores but some drives do have a problem with a certain type of copy protection.
If however it is not that, and there is no copy protection see if you can back the first disk up on another pc and try the back up disk in yours to see if that may help.
Tosh those files should of been on the OS CD.