Why a primary drive and data drive?

You install the OS on the SSD, then you install all programs or files on the Hard drive. If you're building a computer, and you use an Intel Z77 chipset, you can use Intel Smart response, which combines the Hard drive and SSD into one fast drive.

No reason at all. this has become a fashion since serious operating systems like Linux have been seen to use partitioning or individual drives. under Windows it is generally not a particularly useful arrangement. Wherever you load programs under Windows the largest part of most of them, the library files and registry entries must go to the system drive. If the system drive or partition is too small it is easy to run it out of space in a very short time. You may get a performance gain by using different physical drives as each can be reading and writing at the same time, but from experience this is minimal.

Its the most cost effective way for performance.

You can buy a SSD and put everything on it (but you will need like a 240GB+ SSD, thats like $250-300+)

Or you can buy a SSD ($60-150) + HDD ($100) and get more storage space, with the same performance of a 240GB+ SSD

This is what you do.

install windows and any software/games that use use alot onto the SSD. Then put EVERYTHING else on the HDD. This way with windows and everything you use alot is fast, and things like music and videos and etc which gain no benefit from a SSD, be be stored w/o wasting space.

Most of everything needed is on the windows installation (lots of things depend on the windows registry and windows libraries, so having them on the SSD, drastically increases overall computer performance). You can also boot up windows in like 5-10secs.

Then with any room left (windows and etc will use likely 20-25GB), you can put any programs you use alot (maybe photoshop) on there to run faster, or any games. But it really depends on the game(s) as they vary in size.

Games benefit from SSD (well not only from the overall boost from windows being on it), but also will load the game faster when you open it. It will load maps/levels faster, AND you will get no drops in fps when needing to load game data from the storage drive. (as sometimes games need to load a bunch of data while playing, wont get into the technical too much, but when that happens you will either get a drop in fps, or even freeze for split seconds while it does that. Anyway a SSD eliminates that for games installed. So competitive games are definitely good to use on a SSD.

Also as colonic said, both hdd can actually be accessed at the same time aswell, which just increases performance that much more.

normal 7200rpm hdd = 80-120MB/s read/writes

sata III SSD = 500MB/s+ read/writes

most people go for 60 or 120GB SSD with a 1TB HDD.

**i can give you more detail on how to set it all up if you are going to buy one, shoot me an email through my profile and ill help you out with other tips/tricks. Id put it here, but i think im already close to out of answer space, damn yahoo :3

A primary drive is for all your music and photos etc and a data drive is storage for all the system files such as windows 7