Hard drives and types -- what makes them so different?
The list of differences I managed to compile while shopping for Christmas was:
Blacks supposedly have dual processors, which apparently gives higher throughput and lower latency. They also have a five-year warranty.
Greens aren't 7200 RPM drives. Western Digital lists them as "Intellipower", which they originally claimed meant that they varied their speed dynamically from 5400-7200 RPM to help save power when not under heavy use. These claims were quietly replaced by a very fine print definition of each individual drive is manufactured with a slightly different speed. Independent testing has confirmed that they are ~5900 rpm drives. They also have a reputation for a high failure rate, especially early on and have only a two-year warranty. They have a "feature" where they park the heads after eight seconds of inactivity supposedly to save a little more power, but this puts more wear and tear on the drive and people generally recommend disabling it. Bottom line: stay away from these drives.
The Red edition is just a Green with the Time Limited Error Recovery (TLER) feature enabled in the firmware, and a little bit more QA before they go out the door, with a three-year warranty. TLER only matters for use in RAID arrays and really doesn't seem to be very important to other users.
The "RE" RAID Edition are black drives with TLER and a five-year warranty, and high price. They are aimed at corporate servers that are always on and are always accessed.
The Blues are the run-of-the-mill average. I ended up getting three of these and put them in a RAID 10 for both speed and reliability, for less than the price of a 10,000 rpm VelociRaptor. The array pushes ~500 MB/s.
Other factors to consider include interface speed, platter density, speed of the cache, and the design of the drive controller.
Blacks also have support for various commands (for example, TLER) that are important for enterprise users (RAID storage, etc.), but generally not useful to the home user.
Western Digital has a list of the features unique to Black on their site.
To answer the first part:
7200RPM hard drives
and types.
What makes them so different?
Nothing. There is nothing special about 7200 RPM drives.
They are often faster than their lower RPM drives because most mechanical hard drives store their information on a rotating platter. Spin the platter faster and it passes faster between the point where the information is read or written.
If you take a platter from a 7200 RPM drive, slow the drive down to 5400 and re-low level the drive then you will end up with a drive reading and writing at ¾ of the 7200 speed drives.
(5400/7200 = 3/4, or ¾).
It will also have a slower random access time since to access a piece of random data you need to wait an average of half a platter rotation. If the platter moves faster then this takes less time.
Is 7200 RPM special?
No, drives have been produced with all kinds of rotation speeds, platter densities, number of platters etc etc. It is just that lots of the recent drives are either 5400 RPM or 7200 RPM. Those 7200 RPM's often get other beneficial features while the 5400 RPM often get aimed at low power, low noise environments.
That is not to say that those are the only speeds. Drives I have used in the last 6 years included rotation speeds of 4200 RPM (bigfoot), 5400 RPM (original from my laptop), 5900 RPM (my newest 4TB drive) and 15k RPM (a small SAS drive in my desktop).
Higher than 15K RPM does not seem to be used. Probably because the added effort to does not warrant the performance gained.
Lower than 5400 RPM is also rare. Usually only in ultra-low-power drives or in drives with large platters (e.g. the bigfoot series which was the last 5 ¼ consumer drive I saw).
Still, even if it is used a lot it is nothing special.