Why in the world would it erase your external hard drive? I'm sorry, I just don't understand these types of questions.
Simple answer: No, not unless you explicitly tell it to by launching the Disk Utility program and walking through the several steps and warnings to manually erase the drive. No OS X update (released to the public via DVD or download) has ever required you to erase your hard drive in order to install it, or done such an operation without the user seeking out and choosing that option within the installer's steps. In Snow Leopard, Apple made it more difficult, not less, to accidentally erase your hard drive by removing the option from the Installer.
jW