I'm sure this is a stupid question. But is iCloud storage just a backup of what's on my computer, or a separate drive? That is, if I move my Photos library to iCloud, are the photos off my computer? Does my computer access the photos on iCloud as if iCloud was an external drive? Similarly, if I move documents onto iCloud, is it like they're on an external drive? Or is iCloud just a back up of what's on my computer? Are there relevant settings?