How to install Yosemite correctly
Three options await for installing OS X Yosemite. The first is to install over OS X Mavericks. This has the advantage of forcing you to use Yosemite on a daily basis, but it’s risky (and reckless on a critical work machine), since beta software can be buggy.
If you’re dead-set on it, we strongly recommend taking at least one full clone of your pre-Yosemite Mac to an external drive, using Super-Duper! or Carbon Copy Cloner. Boot from it to check everything works, and this will provide an escape route if you find software compatibility issues when using Yosemite.
The other options are safer, and involve fresh installs to a partition on your Mac’s internal drive or to an external drive. A disadvantage is you’ll have to boot into Yosemite to use it; but in keeping everything isolated, your existing data should remain safe.
Which fresh-install option you go for depends on the kit you have available. Mavericks needs a minimum of 15GB of space, but it’s recommended you have about 50GB for properly testing it. If you’ve a spacious and somewhat empty main drive in your Mac, you can create a new partition in Disk Utility. The process is nondestructive and safe, but we nonetheless recommend backing-up/cloning your Mac first, just in case. When you’ve done that, select your drive in Disk Utility, click Partition, select the main partition, and click the ‘+’. Choose an appropriate size for your Yosemite partition and click Apply. When Yosemite is fully released, you can delete this ‘test’ partition to recover the space.
External drives can be slower and not give a realistic impression of the OS running on them in terms of performance. However, Thunderbolt and USB 3.0 drives should be fast enough, and all external drives are easy to wipe clean once Yosemite ships. If you’re using a new drive, format it in Disk Utility, using the GUID Partition Table setting under Options in the Partition tab.
The process for installation is as per recent versions of OS X: double-click the installer app, follow the instructions and wait. If you go for a fresh install, you select which system to boot into via Startup Disk in System Preferences; alternatively, hold å during start-up to access Startup Manager and choose a volume to boot from.