I wouldn't be surprised if they shuffle games between external drives and the SSD _almost_ on demand. The HDD might not give the random access and burst speeds new games need, but if they can maintain 100MBps they can transfer a 100GB game in about 15 minutes (and faster if the game can prioritise the assets it needs or boot before it's fully installed). That's not the end of the world - I've had longer patches on PS4.