Not to be a drag, but I'd be very surprised if you were able to squeeze 40 megabytes/second through that USB 2.0 interface. Sure, the spec says you should be able to do 60, but real world, I'm a little skeptical. I'd think 20-25 is more likely (due to USB packet overhead).
Ah, here we go again... more 'b' versus 'B' confusion.
USB 2.0 Hi-Speed is rated as 480 mega BITS per second. ATSC (here in the U.S.) specifies a maximum stream rate of 19.2 mega BITS per second.
With the original poster's HDHomeRun streaming up to two such programs, that's only 2 * 19.2Mb/s or 38.4Mb/s. Add in a third stream a "media extender" will often be "watching", and you are up to 3 * 19.2Mb/s or 57.6Mb/s (though that last stream is traveling in the opposite direction of course).
Just make sure what ever file system you install on that "drive" is friendly to streams. I'd suggest a "cluster size" of 64KB. This will keep even these three streams able to utilize the highest throughput of that "drive" (i.e. random I/O rate using 64KB-consecutive blocks is nearly the sequential/maximum I/O rate of most hard disks).