USB HDDs and Safepoints. Am I missing something?

You don’t lose anything. If you’re keeping data on your pc, then you can just connect the external drive on to the My Cloud and use it for extra storage.

In order to use Safepoint, you need to go into the dashboard and turn it on. If you haven’t done that, then you can just connect the drive and it will show up in the My Cloud as another share that you can map if you want.