Um... You simply posted 2^64 in bytes, which is not the limit of a 64-bit file system. The correct size limit would be 16,777,216 Peta-bytes (18,889,465,931,478,580,854,784 bytes and 1,024 times as much; yes, I don't know the prefixes past peta-). You forget that a single standard FAT cluster is 8kB (8192 bits), not a single byte. Anyway, using a 1kB cluster is out of the question, as you'd likely lose 70% of the storage space on a drive with such an insanely small cluster size.
Also, it amuses me that most companies simply get away with claiming that a gigabyte is 1,000,000,000 bytes (or that a terrabyte is a trillion bytes). I think the only reason for the decimal prefice is due to how convenient that 2^10 is fairly close to 1,000. (only 24 above)
As for the drive itself, from the information regarding buffers (the fact that it has four 8MB buffers), I would guess that it is merely four 3.5-in. 400GB HDDs, not a impressive. And look at the weight of that thing! (5kg) A single normal 3.5in. HDD weighs slightly less than a single kg; that's a lot of weight for that "huge" HDD.
Also, I could see how useful such a drive would be. Considering that it is external (which, by the way, would almost double the price, for those who think it to be too expensive for a HDD), it would be an ideal fast-writing backup medium for collections of servers.
Wake up, George Lucas... The Matrix has you...