When using certain USB drives as the installation medium, the debian installer complains with: "No common CD-ROM drive was detected. You may need to laod additional CD-ROM drivers from removable media, such as a driver floppy. If you have such media available now, insert it, and continue. Otherwise, you will be given the option to manually select CD-ROM modules. Load CD-ROM drivers from removable media?" Answer No; then you're prompted to manually specify a CD-ROM path. Say yes, and say that no module loading is required. When prompted for the device path, press Alt+F2 and ls /dev/ as it suggests. The installation drive is probably the last hard drive, e.g. if you see /dev/sda and /dev/sdb, try /dev/sdb. If you get an error about "unable to install additional packages from CD-ROM" later in the install, choose "go back" and choose a package mirror to download from.