2. I can do it on Genos.
No you can't do what you plan (load sounds directly from HD) even with Genos!
The answer provided Fred concern ONLY the amount of flash memory (although as already stated by RoyB, you can install a 2GB flash memory on Tyros 5).
You made an "investment' (buying Tyros-5) without double checking if the keyboard was able to do what you want...
Yamaha use this technology for all the recent keyboards.
I don't know why (I also don't care, personally).
In any case, as far as know, even the faster HD can't help on "sound loading". Not in Tyros or Genos but in any synth, arranger, keyboard etc..
In most cases there is the use of RAM memory (flash memory) involved.
You load from HD to RAM and then play. Even computer VST's works this way.
There is always the need for a very fast buffering memory in order to play along the keyboard.
Even "streaming" is actually the reproduction of small chunks of data, but not directly from HD, always there is a buffering memory.