Here is a quick tutorial how to start Debian and probably other Debian based distros from a USB Hard Disk. With USB Flash Drive usually there is no need for any custom actions.

1) Once the installer complains for missing CDROM and asks to load CDROM drivers select NO:

2) On the question “Manually select a CD-ROM module and device?” select YES:

3) On “Module needed for accessing the CD-ROM” select NONE:

4) On “Device file for accessing the CD-ROM” type /dev/sda4, or if that doesn’t work try /dev/sdb4, dev/hda4, /dev/hdb4 etc. depending what device name the USB disk got:

You can get the available device names by pressing ALT+F2 to enter a new console and type:
ls /dev/
or
dmesg | grep disk

Just remember, it has to be the fourth partition, e.g. /dev/sda4.
Next press ALT+F1 to return to the Setup screen.