If you have not yet done so, download and install GoodSync.
To set an external drive as a source or destination, follow these steps:
1) Open GoodSync and select or create your Job.
2) Click the Left or Right Browse buttons to view the files and folders available for selection.
3) Choose the "My Computer" option from the list.
4) Click "Local" and in the list that drops down you should see your external drive listed. External drives will list with a drive letter.
5) Click your external drive to select it.
After completing these steps for the Left or Right folder, you may select the folder or external drive on the opposite side and proceed with your Job.