“Even though OS X Snow Leopard has been around for quite a few years, it remains a favorite for some Mac users with legacy software support,” Andrew Kunesh writes for Tuts+. “Additionally, some older Macs stopped receiving updates after Snow Leopard, forcing IT departments to continue support.”
“Because of this, it’s important to keep a bootable Snow Leopard install disk around if you have a Snow Leopard machine or manage them,” Kunesh writes. “However, there is no automatic way to create this drive on your Mac, so I’ll show you how to do so in this tutorial.”
“Before starting this tutorial, ensure that you have a USB drive with at least 8GB of storage as well as a Snow Leopard retail DVD or disk image,” Kunesh writes. “Grey DVDs that were included with a Mac at the time of purchase cannot be used to make USB boot drives as they do not include drivers for Macs other than the computer it was shipped with.”
Read more in the full article here.
Great. But where do you get a Snow Leopard Disk Image?
You buy it (a physical disc) for $20 from the online Apple Store
http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard
Plus the Mac needs to be old enough to start up using Snow Leopard. If that Mac happens to have an optical drive, you don’t really need a USB version of the installation disc (what this article is showing how to create), except when supporting a LARGE number of Macs running Snow Leopard, where using a USB flash drive would be more convenient than the disc.