If you have an older PC you may have double-density floppy drives, your PC will not be able to use the newer higher density floppy disks.

You may find that your PC can read and write to floppy disks just fine, but when you take this disk to another PC, it can't read the floppy disk. If this is the case, your floppy drive could be going south. The read/write heads could be out of alignment and causing this problem.

Finally, you may just want to add another floppy drive to your system, so you can more easily copy from one floppy disk to another.