It's not possible. I mean it's not like they'll spill over. HDD isn't a cup of water :3 Whats more, portable HDDs are getting to be cheaper and cheaper, so it's not such a big problem where to put all that data. The problem might be book prices however
But that's why we have Gutenberg and MobileRead
Anyway, having a lot isn't about reading them all, but having a varied choice.