USB booting :: USB Install - Protecting existing Hard Drive data

I want to install DSL to a USB Flash Disk.
I've got the live CD up and running fine on my MSI laptop.
When I go to install it to the Flash Disk it asks me for the device name, Example sda:
I have no idea what device to type as I am a linux idiot.
I want to ensure I'm not typing my Hard Drive ID or something.

Where can I find out what the ID of the flash disk is and also confirm that i'm not writing to the Hard Disk?

The harddrive is usually going to be hda, and i'm guessing usb drive is usually sda.
The "a" signifies which physical drive it is...second and third harddrives would be hdb and hdc respectively.

You can use the command dmesg|less to view a log of boottime kernel messages

original here.