2 in 1 HDD that is mixture of SSD & conventional drives mechanism

WD Internal Drives Internal Drives
2

Check out Silverstone’s HDDBoost:

http://www.silverstonetek.com/product.php?pid=245&area=

Also, the Z68 chipset now supports SSD caching:

http://www.tomshardware.com/reviews/z68-express-lucidlogix-virtu-ssd-caching,2888.html

And, I believe Seagate’s Momentus XT already does exactly what you’ve described

(but, I don’t know if it’s OK to mention Seagate products here;  Moderator, please advise):

http://www.seagate.com/www/en-us/products/laptops/laptop-hdd

Hope this helps.

MRFS