Q: Deleting pictures from a hard drive after Photo update

You will have to delete them from each library separately.

EDIT: also remember to empty the Trash in iPhoto & delete the items from the "Recently Deleted" album in Photos.

(I'm aware that Photo pulls from the same library as iPhoto- but how do I delete things from that library?!)

No - this is NOT the case - each library is totally separate with no connected to the other - both libraries are hard linked to the original photos so they are not duplicate during the migration process - there is no shared library and no way to delete something form both libraries at the same time

this artcle describes the linking - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

You can delete either library if you no longer want it although generally it is best to keep them both for now

LN

yes- when the last hard link is removed the photos will be deleted

LN