Install Mavericks on second partition in Yosemite-only system? I have a 2012 MacBook Pro (so yes it's capable of Mavericks). I have only Yosemite currently installed on the drive. I have an app I need to run that does not seem to properly support Yosemite, so I want to install Mavericks on a second partition to setup a dual boot environment.

I do not presently have access to a spare USB drive that can be erased to create installation media nor do I have any blank double-layer DVD discs.

I have already used disk utility to resize and create my extra partition. However, I am not able to actually start the OSX Mavericks installation. The error is "This copy of the Install OS X application is too old to be opened on this version of OS X."

I need to get this done for a college project due in the next couple of days and won't have access to a flash drive that can be erased and used for install media within that time. Is there anything I can do, say from the terminal, to force the installation of Mavericks to proceed onto the second partition?
You have to cheat the Mavericks Installer by running it on a "pre-Yosemite" system.
The system version is determined by the key "ProductVersion" and the associated string in the file SystemVersion.plist. Replacing the 10.10.x string with 10.9.x or 10.8.x will do the trick.

Beware: After manipulating said file you can't open most of the current apps anymore. 

Prerequisites:  


a current backup of your Mac
already downloaded OS X Mavericks Installer
a Mac capable of running Mavericks


Age your system:


Log-out any other user and log-in with your admin account.
Quit any running app and open the folder containing the Mavericks installer.
Open Terminal.app.
Make a backup copy of SystemVersion.plist first by entering:

sudo cp /System/Library/CoreServices/SystemVersion.plist /System/Library/CoreServices/SystemVersion.plist.backup


and hit ↵
enter your admin password and hit ↵ (the password won't be visible!)
Modify SystemVersion.plist:

sudo nano /System/Library/CoreServices/SystemVersion.plist




change the both strings 10.10.1 (or 10.10.0) to 10.8.1 or 10.8.0

write the changes to disk with ctrlO followed by ↵
and exit nano with ctrlx
Recheck the successful change by entering:

cat /System/Library/CoreServices/SystemVersion.plist 

Enter exit, hit ↵ and quit Terminal.app.
Start Install OS X Mavericks and choose your already created second partition to install to. You may have to click "Show all partitions" if your second partition isn't visible initially.
After installation and configuration of Mavericks is finished revert the changes of SystemVersion.plist on the Yosemite partition:


Rejuvenate the Yosemite system:


Open Terminal.app in your Mavericks admin account and enter:

sudo rm /Volumes/NameOfYourYosemitePartition/System/Library/CoreServices/SystemVersion.plist


and hit ↵
enter your admin password and hit ↵ (the password won't be visible!)

then enter

sudo mv /Volumes/NameOfYourYosemitePartition/System/Library/CoreServices/SystemVersion.plist.backup /Volumes/NameOfYourYosemitePartition/System/Library/CoreServices/SystemVersion.plist


and hit ↵
Enter exit, hit ↵ and quit Terminal.app.


If the install fails and you are urged to boot to Yosemite (which probably fails), try to boot to Recovery Mode or Internet Recovery Mode, revert the changes of the file SystemVersion.plist on the Yosemite volume from the command line in the same manner as described for the successful Mavericks install in step 10.
Then restart from scratch.

(Tested in a Yosemite VM...works)