Wow! That would be the HDD I will put inside my PS4. I don't mind to pay more and have it preinstalled. If not, I really appreciate if Sony will let us change the HDD as the did with the PS3 (pre-slim).
But IMHO, they won't let us change the HDD for anyone we want. The PS4 price tag will be very tight, so I'm sure they expect to win money selling their own HDDs. I only hope they offer big ones.
I own a PS3 with a 1TB HDD I bought, so I expect that size as minimum.
"Fusion drive" is just a fancy name for SSD caching - something Intel calls "Smart Response Technology" (available in Windows laptops everywhere). It works by using two drives inside the computer, an SSD as well as a regular HDD (3,5" in the case of the iMac) at the same time. We won't see this type of expensive technology in next gen consoles.
There are no 2,5" drives available on the market that are bigger than 1 TB and of standard 9,5 mm build height, that is the absolute maximum - therefore a 250 GB/500 GB split makes absolute sense. If we can actually change the HDD ourselves, I'll be perfectly happy with that. No drive bigger than 1TB is possible for launch.
I'd hope that they have a 2TB model, preferably I would love if that had that
3TB Fusion HDD that the new iMac's have, where 256GB of it is SSD and the rest is for storage.
"Fusion" is a marketing name, not a real physical object.