Greetings mattchu,
Thanks for being an Apple Support Communities member and for taking the time to ask this question! We’re glad you’re here and can’t wait to hear more from you! :)
It sounds like you are not able to boot from your external HDD, which has Mac OS X Snow Leopard on it, using your new Mac which is running Mac OS X 10.10 Yosemite. These type of issues can occur if your new Mac shipped from Apple running a later version of Mac OS X than 10.6 Snow Leopard.
The information in the following articles explains this information in more detail.
Starting from an external USB storage device (Intel-based Macs) - Apple Support
Intel-based Macs support starting from an external USB storage device's volume that:
- Has been formatted with a GUID partition type
- Contains an installation of Mac OS X 10.4.5 or later, or Mac OS X 10.5 or later, which is compatible with (or shipped with) the Mac that the USB device is connected to. Note: You should not use a version of Mac OS X that is earlier ("older") than the version your Mac shipped with.
Use the version of OS X that came with your Mac, or a compatible newer version - Apple Support
We hope that these articles were helpful to you in explaining what may be causing this issue. We invite you to reply and let us know if this is the case or to provide any further information. We are here to help!
All the best!
