[dnanian]

Well, in general it's a very good idea to have *both* a backup and a safety clone, especially when you're making a big change like this.

In your situation, since you don't really have the space to do both (unless you have another drive), I'd suggest making the backup and upgrading to Tiger on the internal drive. The "back out" would be an archive-and-install of Tiger (to retain your data), or a restore (which would lose what you've done since the upgrade). But, it's unlikely you'll run into something that's not fixable at this point.

(Check at Macintouch that any important programs you're using are Tiger compatible, and upgrade them *before* you upgrade to Tiger, though!)

So -- in sum, don't just have a Safety Clone. If you have to choose, make a backup, and upgrade the main drive.