It depends on what the exact issue is. Did you double check that you inserted the correct disc? If you are not installing all scenery at once, it may not need every disc, and the installer will not warn you if you insert the wrong one.
If it is an input/output error, quit the installer. Insert the problem disc & open it in a file explorer window. Try to copy the X-Plane folder to the desktop. If that doesn't work, it indicates the disc is now defective.