How do I sync to a local drive without having the contents of that drive uploaded as well?

The short answer is that you cannot. SpiderOak requires that you backup the data before you can synchronize it; however, given our intense de-duplication, adding a local drive to your backup (if it has the same data contained on your local hard drive or other computer in your SpiderOak network) will not use up additional space in your SpiderOak account as the data will simply be referenced to the external device.