HDD buffer with laptop RAM slot

[#1]

One thing I have been wondering for a long time is why the HDD have such a small buffer. I would love to see a Hard Drive with a RAM slot, prefereably a known type such a s laptop DDR2 RAM slot for its buffer. I hate OS disk cache solutions, and having the ability to decide to increase the physical characteristics of my secondary memory devices would be great. I could decide then to get more RAM for my primary drive without hurting the cost in my other drives. Hey, I could move the memory if I wanted to. And this could double the functionality between buffer and cache. If the drive starts and immediately stores some of it most accessed content in this RAM, it will be a much faster solution than existing systems. (in reality you could store the statistics in a small flash memory).