Media changed: please insert the disk labeled 'Debian GNU/Linux

I installed Debian 7 using a USB flash drive "burned" from the first DVD ISO of the Debian installation CD/DVD. I did a very minimal install without Debian desktop environment, Print server and Standard system utilities. After installation and a reboot, I was presented with a console with the words *Debian GNU/Linux 7 hostname tty1*. I supplied the login username and password. After I typed the command sudo apt-get install xorg, an error message popped up stating: Media changed: please insert the disk labeled 'Debian GNU/Linux 7.4.0 _Wheezy_ - Official amd64 DVD Binary-1 20140208-13:47' in the drive and press Enter I inserted the same USB flash drive into the same port and after waiting for a few seconds, I pressed Enter. The same error message popped up. I have tried the following steps on the advice of some of my colleagues: 1. remove/delete all the entries in /etc/apt/sources.list and reboot the computer 2. dmesg and fstab show that the USB thumb drive is mounted on /dev/sdb1 3. sudo mount /dev/sdb1 /media/usb0 4. sudo apt-cdrom -m -d /media/usb0 add After doing the above, the following error message appears: Using CD-ROM mount point /media/cdrom/ Identifying.......{a long string of alphanumeric characters} Scanning disc for index files............... Found 0 package indexes, 0 source indexes, 0 translation indexes and 0 signatures W: Failed to mount '/dev/sr0' to '/media/cdrom/' E: Unable to locate any package files, perhaps this is not a Debian disc or the wrong architecture I prefer to install Xorg (60MB) and gnome-core (400MB) from the USB stick. The NGO that I am working with is in a developing country with a very basic internet access infrastructure. Internet access is very patchy and the average download speed is less than 2 Mbps.