Harddrives can not read or write higher than 150MB/s. The only thing that goes higher, is burst speeds which is to the cache chip. That's rather useless, as its much smaller than your 4GB RAM which also acts as cache.
HDDs don't need much cache memory; they need about 1 or 2 MB to allow them to do 'write-back'; that means they store all write requests the HDD receives to its memory chip instead; so it builds up a queue of write requests to do. This 'queue' makes sure the mechanical part of your harddrive can continue to work.
Thus, any more cache memory is mostly marketing. The benefit is extremely low; cache on your CPU now THATs important but not on your HDD. Don't trick yourself by the marketing!
So HDDs do about 100-140MB/s; SATA/300 is more than enough for those. Even SATA/150 wouldn't limit their speed to any noticeable degree.
Now the RPM part, which is more difficult:
low rpm, high density (2TB 5400rpm): bad as system disk (random access) - good for large files (throughput)
high rpm, low density (300GB Velociraptor 10.000rpm): good as system disk (random access) - worse for large files (throughput)
As you can see, the 10K rpm Velociraptor isn't that good for storing large files; such as movies, music, archives, whatever. It could be even slower than a 5400rpm disk; how can that be? Well, higher rpm does mean higher performance; but only if all other factors remain the same. That's not the case with the Velociraptior, as its data density is less high than the 500GB/platter 5400/7200rpm harddrives out now.
Platter density is probably the most important spec for a harddrive; you could instantly see how 'advanced' the drive is and to which generation a HDD belongs by looking at its density. HDD makers don't make it easy to find this; it's buried in the specs. For example:
500GB/platter: current generation
333GB/platter: previous generation (still being sold)
250GB/platter: old generation (still being sold for disks 500GB and below)
200GB/platter: etc..
So the primary specs to look at are:
1. Platter density (2TB should be 4 x 500GB platters = 2000GB)
2. rotation speed (5400rpm best for: large files; 7200/10.000: random access)
3. Sector size (512 = default; 4K means Advanced Format drive which could be more reliable but requires aligned partitions; not suitable to Windows XP)
That's it really, forget the SATA generation, forget the cache chip size.
I said 'the lower the rpm, the better' since lower rpm means lower power consumption; less heat. And still good data throughput. The access times will suffer though; so a low rpm HDD would be a terrible system disk; but still a good mass-storage disk for large files. 7200rpm disks require cooling and have lower lifetime due to them rarely being cooled properly.
Temperature is not that important to HDDs; it is temperature variation that kills HDDs; one part being cooled while the other not could create an imbalance; and quick temperature changes are what wears on a HDD. It will contract and expand the metal inside the HDD tearing it apart over time. That's why i recommend 5400rpm disks; you don't need to cool them and because of that will have very modest temperature changes and perfect heat spread over the surface because you're not actively cooling them; heat will travel uniformly and dissipate slowly.
Sorry for the long message; but hey you don't read this info everywhere. Some may argue some points i made; but i think this pretty much sums it up in an understandable way.
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