CD and DVD is write once, read only so to pull off the old CD/DVD as a floppy (read write) there's either an app you installed to do that or still an app plus CDRW or DVDRW media.

It's also possible the media hit a defect and went read only to block writes and prevent data loss.

You need to know how you enabled this in order to know how to fix.

HOWEVER this method is now considered unsafe and not to be done anymore. The new way is USB memory sticks but still with all the progress NEVER run such a system without backups of what you can't lose.