3. Explore Yosemite and report back to Apple
Apple's goal with this program is largely to solicit feedback about Yosemite, particularly about its new UI. If there are things that aren't working, are confusing, seem like they should work better, or that you really dislike, you should report them to Apple using the Feedback Assistant that's included. During the public beta prior to the release of OS X in 2000, Apple made several changes based on user feedback -- the most obvious was restoring the Apple menu in the shipping version of OS X, which had been replaced with an Apple logo in the middle of the menu bar.
Although Apple suggests simply using your Mac as you normally would, you should also explore a bit. The advantage here for Apple is that you'll play with more Yosemite features and potentially provide more useful feedback. You also might discover new (or existing) features that you might not have considered using otherwise.
Apple wants to hear what testers think about Yosemite via the built-in Feedback Assistant.
You should also report issues with third-party software if it seems that Yosemite has broken something in existing apps. That gives Apple the ability to look at underlying problems affecting those particular apps and potentially others.
4. Don't install Yosemite on a mission-critical Mac
You shouldn't install the Yosemite beta on a Mac that you need for important work. There's a very real possibility that you may encounter a serious problem -- like losing important data, having some key apps stop working or having your entire Mac be disabled.
Let me repeat: Don't install the beta on your primary Mac -- the computer you use day-in and day-out for work or personal tasks. You should install it on a secondary Mac that can easily be wiped clean and restored if need be.
5. You can install Yosemite on an alternate drive or a virtual machine
If you only have one Mac or you decide to ignore my advice and install Yosemite on your primary machine, you should at least consider putting the beta on a drive that isn't your typical startup drive (usually the internal hard drive or SSD inside your Mac). You can install the beta on an external drive, a second internal drive (if your Mac has one), or a hard-drive partition. You can then boot from that drive or partition when you want to use the beta and boot up from your primary startup drive when you need to get things done.
Installation on an alternate drive reduces the risk that a catastrophic failure will befall you or your data, but it isn't a guarantee. An issue that impacts your Mac's file system could affect both the alternate "Yosemite beta" drive and your startup drive. The most likely scenario in this case would be that some type data loss that hits all data on both drives.
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