A few thoughts:
There are obviously bigger drives available so I will assume space is not your main concern here. You can however buy an external RAID cabinet if you simply want more drive spindles. Some of the larger USB-drives on the market are actually build like this (effectively 2 striped drives)
Some drives comes with a USB port for chaining, just beware of power consumption and there will obviously be a performance hit on that bus.
The actual impact depends on how often then require data at the same time etc.
Hubs can work sometimes, just make sure you buy a quality one from a well known brand. It usually comes down to witch chip they use. It´s not ideal though.
The dongle is accessed every now and then, but the hit on your USB-bus is tiny. I have multiple on one HUB and it works great.
The best configuration depends on the nature of your projects. The only time you really need a separate drive for libraries is if you´re running multiple tracks of huge libraries. I used one drive for years (not now though), and that always worked for me. Adding more RAM really helped.
The number of audio tracks dosen´t say much as they may just be smaller (repeated) loops that will be cached. A large number of longer and continuous audio tracks is another story. You´ll have to simulate some load and monitor your disk performance to determine the best configuration.