There are a number of reasons that you may want to have separate drives, , You may want to run your operating system on one drive your flight Sims on another drive and your mass storage on another separate drive. The first reason is performance.Having separate drives can allow you to select drives that best meet your needs. For example you could use a faster drive like a velociraptor or maybe even a solid state drive (SSD) for your operating system, another fast drive for your fight sims and then a mass storage drive for your music and general files. The second reason is isolation Some people like to isolate the operating system (OS) on a separate drive , They feel it will lessen the chance of the drive being corrupted because only files relating to the OS are stored on that drive and may allow them to reload the drive quickly should the OS crash. On new systems it's very popular to install your operating system on a solid state drive, SSD's are very fast and because the solid state drive has no moving parts there is less chance of failure. Same goes for your flight sims, If you install them on a SSD you can expect much faster load times and faster drive response. Because SSD's are expensive and generally have smaller storage capacities Many people will use a 40GB SSD like the Intel X25-V for the operating system and then a separate larger SSD for the flight sims, They will then use a third separate drive for mass storage, 1-2 terabyte drive are very popular for mass storage and are much less expensive than a SSD on per GB basis. If SSD' s are out of your budget or wont work on your system, then you may consider a drive like the 10,000rpm Western Digital VelociRaptor . The velociraptor is very fast and strikes a good balance between price, performance and storage. You should also note that there are exceptions to every rule and there are some very fast 2TB drives available. You will need to do your homework and determine what drive(s) will work on your system, what type of storage you need and what your budget will allow. Video for more about SSD vs Sata drive performance.
Hope that helps Cheers Data