mount -t vfat /dev/sdc /cdrom
type 안 해주면 invalid argument error
cd-rom 마운팅이 안되서 세번째에서 usb로 ubuntu 설치가 막혔을 때
I had the same problem as you, but solved it by choosing the option "Start terminal" (I'm translating from swedish, so I don't know exactly the name, but the option is placed above the "Abort installation entry" in the Debian installation-program.
From the terminal I did mount /dev/sda to the /cdrom folder:
mount /dev/sda /cdrom
then returned to the debian installation-program by writing:
in the terminal.
Edit: As is it not so obvious, this comment is about the installer not finding the cdrom when installing from USB.