ok, I understand now your post and your problem.
you have a usb flash memory with a installer of Debian. Your computer think your usb is a cd-rom.
your usb flash memory cannot be open for writing a file.
So, you probably know by now this, but when you format your usb to use it as an installer, the program that format your usb tell to the usb to behave it as a cdrom.
The solution of jefro is giving is multiple:
do you have another usb to copy the file?
if not, can you make a partition in your usb to have 2 usb in one? One partition will host the installer and will behave as a cdrom where you cannot write and the other partition will behave as a standard usb where you can write.
Does it solve your problem?