Since it's a proprietary solution that would need to have fixed minimum speeds across all variants & setups I think we can assume the solid state storage is acting as a second level cache for the system memory, and from info so far that system memory looks like it'll be split somewhat too. I don't think there'd be much benefit in going beyond 128GB with how they've demo'd it so far, seems like it loads all the games media on initial launch to speed up in-game world load times & streaming, even 64GB would be more than enough for most titles atm since you don't need all the games media loaded in. Obviously, allowing users to access it as a storage device would make the use cases they've demonstared so far more or less impossible to guarantee for devs if a user wanted to use a USB device or similar for storing more games, so I don't think it's at all possible this will replace HDDs in any way.