Does Mavericks allow an install to a different drive than the current startup drive? I am running Mountain Lion on a 2008 Pro Desktop and would like to try Mavericks without losing the old OS. I'd first like to see what apps run fine vs which ones I'd need to update. And also see the performance before fully committing.
Yes, you can. Download and run the installer from the app store, click continue and agree to the license. In the screen asking for where to install, click the show all button to show drives other than your startup drive. You can select either an empty drive or one with older versions of OS X installed.
Since you are very likely to install OS X Mavericks again, you can make a copy of the installer to avoid having to download it again. Make a copy of the Mavericks Installer app in your Application folder before the installation begins, since the app will be deleted after the installation. You can use it to install OS X Mavericks on another drive or Mac. See here for more discussion: Re-use Mavericks install file