Hi ThorgrymW,
Thank you for visiting Apple Support Communities.
If your MacBook Pro is only booting to your OS X Mavericks partition, try this method to verify that your Snow Leopard partition is still available and working:
You can change the startup disk your Mac automatically uses from System Preferences.
- From the Apple menu choose System Preferences.
- Click the Startup Disk icon in System Preferences, or choose View > Startup Disk.
- Select your startup disk from the list of available volumes.
The next time you start up or restart your computer, your Mac starts up using the operating system on the selected volume.
If you're able to boot to the Snow Leopard partition, try using this method to access Startup Manager the next time you start up your Mac:
Use these steps to choose a startup disk with Startup Manager:
- Turn on or restart your Mac.
- Immediately press and hold the Option key. After a few seconds, the Startup Manager appears. If you don't see the volume you want to use, wait a few moments for Startup Manager to finish scanning connected drives.
From:
How to choose a startup disk on your Mac - Apple Support
If you are not able to boot to your Snow Leopard partition using either (or both) of these methods, reset the PRAM (also called NVRAM) on your MacBook Pro next:
- Shut down your Mac.
- Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
- Turn on your Mac.
- Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
- Hold these keys until the computer restarts and you hear the startup sound for a second time.
- Release the keys.
After resetting NVRAM, you may need to reconfigure settings for speaker volume, screen resolution, startup disk selection, and time zone information.
From:
How to Reset NVRAM on your Mac - Apple Support
All the best,
Jeremy
