If you are considering signing up for the Yosemite beta, first make sure you make a complete backup of your hard drive, then take a few minutes and read this excellent walk-through by Macworld’s Dan Frakes that’ll take you through the process of creating a bootable install drive of the Yosemite beta.
You did backup your hard drive first, right?
So many of these “how to” articles are an entire bucket of KFC crazy. (See Major League.) This one’s about the same.
How to recreate a bootable Yosemite partition for people who don’t believe in black helicopters but want to protect their data (this assumes your Internet connection is reasonable):
Download the Yosemite beta.
Backup everything. (Duh!) Oh, look! The Yosemite installer got backed up, too!
Physically detach the backup from your computer. (Yosemite probably isn’t going to reach in and smack it. Probably. But why take the chance?)
If you’re going to use a partition, partition your drive using Disk Utility now. It won’t let you accidentally wipe your startup disk while you’re running off it. That’s a good thing.
Double click the Yosemite installer. Let it restart, blah blah. Make sure you install to the right partition/disk. (If you don’t, you’ve got the backup, but that’s extra pain.)
Some people clone, but I just use Time Machine on two drives. If I need to start over from zero and reinstall Mavericks, I’ll use Recovery mode. The Time Machine drive gets reconnected after the (Mavericks) install is complete.