That's taken from the System Photo Library overview, part of Photos' help. This makes much more sense continue reading only if you care about my feelings on the original incorrect wording. That just means you shouldn't use the same external drive for both your Photos library and as a destination drive in Time Machine. The permissions for your Photos library may conflict with those for the Time Machine backup. If a Photos library is located on an external drive, don’t use Time Machine to store a backup on that external drive. Update: Thanks to reader Brian for commenting below that Apple has updated this page with much clearer wording.
