This is exactly what Sony wanted you to do.
This has probably been planned since launch as both Microsoft and Sony seek to ape Apple's success in drip feeding products and tech.
It's rare you see a decent sale on the Microsoft Store but then they've allowed you External Storage for ages so they don't want to bring prices down.
Sony often have good deals on but this has been to fill people's hard drives up so that the prospect of shelling out extra cash for a PS4 Pro doesn't seem like such a bad idea.
Glad they've finally shifted enough Pro units that they finally feel they can release this "patch" that they've been sat on since day 1 to allow PS4 users to add external HD support.
They'll have the Playstation Prime out just in time for Christmas with it's 268 Wiggaflops of S Club 7 Ram and the feeding tube and porta-potty so you never have to turn it off.