Whatever you do, don't leave bad reviews for third-party software--on the Mac App Store or wherever--based on issues with the Yosemite beta. If the app still has issues once the final version of Yosemite is released to the public, criticism is fair game. But it's not okay to ding an app for issues with an OS that hasn't been released.
What do I need to run the Yosemite beta?
If your Mac can run Mavericks (OS X 10.9), it can run the Yosemite beta. In fact, Apple says your Mac must be running Mavericks before downloading and installing the Yosemite beta.
We recommend the same minimum specs for the Yosemite beta that we suggest for Mavericks: at least 4GB of RAM (and preferrably 8GB); and at least 15GB to 20GB of free drive space.
More important, we strongly recommend against installing the Yosemite beta on your primary Mac. Ideally, you'll want a separate Mac you can dedicate to running the beta, so that if you have any serious problems, you won't be stuck without a working Mac--and you can erase the test Mac's drive and start over if things go completely south.
If dedicating a Mac to the beta isn't possible, a reasonably safe alternative is to use a separate drive--a second internal drive, or an external (USB, Thunderbolt, or FireWire) drive--for the beta. Install the Yosemite beta on that drive and boot from it whenever you want to test Yosemite. If you have problems with the beta OS, at least they won't affect your "production" Mac. (That is, unless there's a bug in the beta that affects data on other drives. Which is why you have backups--see the next item.)
Alternatively, if you have a virtualization app such as Parallels or Fusion, you can install the Yosemite beta in a virtual machine. This will let you test the new OS, relatively safely, in a separate window while you're booted in Mavericks.
Whichever approach you take...
Should I do anything special to safeguard my data before I install the beta?
Back up, back up, back up. Unless you're going to install the Yosemite beta on a secondary Mac (or on a secondary drive or partition, or in a virtual machine) and you don't care about losing data on that Mac or volume, you'll want to have a good backup plan--ideally, a separate backup plan from the one you use to back up your main Mac or drive. The last thing you want to do is accidentally overwrite your non-beta backups with data from your beta-test system.
And even if you do install the beta on a separate volume or in a virtual machine, you'll still want to make sure you're maintaining your main backup routine, just in case the beta inadvertently affects other drive or volumes.
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