The short answer is that you cannot. SpiderOak requires that you back up 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.
Syncing to a Local Drive Without Re-Uploading
