This blogger tells us how he uswd a USB thumb drive to install Debian from
Read it here
Some months ago, I upgraded my main workstation. I had reached the point where if I was going to upgrade one component, I had to upgrade them all. This left me with my current new machine and my old machine which was fairly complete and still reasonably powerful. Rather than have a completely assembled computer gather dust in the corner, I decided that I needed to install some kind of operating system. I chose Linux because it was freely available and, well, I like a challenge.
My only problem was that my DVD-ROM drive gave up the ghost long ago and I didn't have a spare to replace it. It occurred to me that since my old computer supported booting from a USB device that I should be able to get the computer booted and the install process started.