Dual Boot OS X 10.9 Mavericks and OS X 10.8
Setting up a dual-boot environment for OS X Mavericks and OS X 10.8 (or even 10.7 and 10.6 if you’re still running an older version of Mac OS X) is easy and allows for a fresh installation of Mavericks to test and develop with. Because the original OS X installation remains untouched, it’s also the safest way to give Mavericks a trial run, which is perfect for running the Developer Previews or if you’re just not sure you’re ready to run OS X 10.9 full time yet.
You don’t need another hard drive to be able to do this, instead you utilize unused space on the existing drive to create a new partition that runs the alternate OS. This process can be completed from a Mavericks boot drive if desired, and using such an USB installer tends to prevent any potential errors with Disk Utility (more on common errors you may encounter at the very bottom of this article), but it is not necessary.
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