It was baffling, irritating, and inexplicable. My wife’s mid-2012 MacBook Pro remains in generally good condition, but she started to bump up against the storage limits of its 500GB drive, between iPhone photos and videos and her music collection. She prefers to not use iCloud for sync and storage, especially after watching me wrestle with problems over the years. (It all works fine for me now.)
A drive upgrade seemed a logical and easy course. Apple used to make it relatively easy to pop a new 2.5-inch drive, and this model wasn’t an exception. You pop a few screws on the underside, disconnect the battery (optional, but sensible), and then remove the drive interface cable and a few mounting and locking screws. Reverse the process, and you’re all set.