Bug#767465: (no subject)

[Jorge Jonathan Moreno Zúñiga <jorgej0805@gmail.com>]

Yes, this happens when installing from a USB drive, the installer
mounts it as a cd-rom and it gets added to your sources file, but once
your system gets installed the usb drive is never mounted (at least
automatically) on the same place, you can solve your problem by editing
your /etc/apt/sources.list file, comment out any line containing a
cd-rom by adding # on the front of it, make sure the online
repositories are not commented, then, do "sudo apt-get update"
and you should be able to install everything from the internet, you
won't be asked again for the cd