[SVass]

Assume that I make a SuperDuper bootable copy of my MB Pro hard disk on an external disk.
Then I boot from the new Snow Leopard or the external hard drive. Can I update ONLY the external drive and then reboot from said disk in order to test my applications without endangering my original computer or disk?

Are there any precautions that I should take (other than having a backup elsewhere on another hard disk)?
Sam

[JellyBeen]

Yes of course. Make sure your external is formatted GUID.

[Simon]

Yes, that will work.

But in order to speed up the process and make sure you end up with a reliable clone, ditch SD.

Instead use the built-in Disk Utility. It's rock solid, it's very fast and it's of course free. You'll find it on every OS X installer DVD and on every OS X installation.
/Applications/Utilities/Disk Utility > Restore.
Select erase destination to get a bootable clone in block-copy mode (fast!).