The most you could do with a external HDD on ps3 was to store save files and music, movies. Thats it. Because of that I dont get why its a big deal. Unless you happen to have 100+ gb's of save data and media.
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I have 6TB's out of 18TB used on my media server I access from my 360.
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You have to install both digital and disc-based games onto the HDD for both the PS4 and X1, so that's where 100s of gb's starts to build up. It looks like games will be taking up 50GB each for the big AAA titles, so that's really not too much space. Luckily, being able to play games as they install on both systems would make it a bit easier to clear space on the drive based on the games your playing.