Now, you're getting 2 things mixed up.
Hard drives, are the devices that read/write data to and from the system. It's amazing that they've progressed as far as they have. Unfortunately, there are only 2 major factors you can improve on in a hard drive.
1. Spindle speed - This depends on motors, and spindle speed decreases the size of the hard drive because the faster you spin it the lower density you need to maintain 100% accuracy in reads and writes. This boosts seek time and transfer rate, at the cost of data density.
2.Ariel density - This depends on the head and disc technologies, and increases the density of the data on the disc. This, while independent from spindle speed, together with spindle speed determines the size of a hard drive, per platter. It also determines the transfer rate of a hard drive, because with higher data density more data can pass under the heads per revolution.
This is the thing that determines the hard drive speed.
The next thing is interface. This is what SATA and SCSI. This is an entirely different beast, and is equivalent to a protocol that allows the hard drive to talk to the rest of the system. This could be regarded as a language, per say.
SCSI - Allows a maximum of 320MB/s per channel, at this time. SCSI is designed for workstation and server use and allows people to hook up as much as 20 devices per channel, and allows each device to queue commands from the system, to move data independent of the CPU *AND* to move data to and from other devices in the channel without dragging down anything else. This is because each device on the chain has it's own controller and thus can move data around by itself without the SCSI controllers intervention. While SCSI is only an interface per se, the hard drives that are designed for this interface have certain characteristics inherent to 'Heavy duty' and 'Work environment'.
First of all, these drives can be more expensive. Since they're for a work environment, where people get tremendous salaries and their productivity or the stability of systems critical to the company is at stake, they can and will afford to use the newest technologies in spindle speed, and since they generally don't need to store massive massive amounts of data, they use smaller platters to enhance seek time (Since the platter is smaller, the head has to move less far to get between the first and the last point on the disc. The disadvantage is that at the same areal density a smaller platter will hold less than a larger platter) as well as faster spindle speeds. The result is that SCSI hard drives hold significantly less than their IDE counterparts. Since IDE hard drives don't have massive speed requirements in the seek time department, they use larger platters (larger platters) and lower spindle speeds, to enhance data density and (consequently transfer rate). The disadvantage of this is that SCSI hard drives also can move smaller chunks of data around at significantly higher speeds than IDE, due to higher spindle speeds and smaller platters, while maintaining equal or significantly higher transfer rates due to the monstrous spindle speeds that they employ.
Serial ATA - Serial ATA was designed to solve a few problems regarding the IDE interface. First of all, the signaling was getting complicated (Needed too high of a voltage) as well as such massive cables, problems with sharing the bus with other devices slowing down the system, the need for command queuing, and much much more. Again, this is only the way that the system talks with hard drives. While it's nice to have every single device in the computer register as a master (something impractical with IDE interfaces) due to the fact that each cable connection is it's own master (Hence the 'Serial' ATA name) as well as smaller cables, ultimately the 'Performance' of the drive is very rarely bottlenecked by the ATA interface. Thus, Parallel ATA Is going to be bottle necked the same way IDE is.
IDE is built to satisfy consumers. Consumers want two things. More storage space, higher transfer rates, and cheaper price. Everything that was important with SCSI, save the higher transfer rates, is thrown out the door. Thus, they compromised SCSI to get the things that IDE users desired.
First of all, the cost was to high to allow controllers on everything so they threw out the SCSI interfaces 'controllers on everything' deal. This makes IDE significantly more stressful on the system than IDE under high workload situations, but who actually puts a huge workload on the IDE subsystem all the time? Very few users would run into the inherent downfall of the lack of onboard controllers.
Second of all, to succeed in more space for less money, they went with bigger platters (bigger so as to increase the sizes of the platters, without compromising seek time too horribly) to store more space. They also put more research into areal density with less emphasis on validation and testing to make sure it was completely safe. As long as it handled well under the normal PC environment with medium to medium heavy workloads it did okay. Normal IDE hard drives will not be subject to 24 hour workloads of 100% utilization so they don't need to conform to SCSI standards.
Second of all, lower spindle speeds were employed, because to the average consumer, trading heat, noise, and smaller sizes for faster seek times does not seem like a good trade. And for many, it isn't. (They don't care about how fast office loads. They want to be able to store more video and have a cheaper hard drive. Spindle speeds increase the cost of the hard drive.) So they went with lower spindle speeds than SCSI (Current generation being 5400 and 7200 versus 10,000 and 15,000 (And soon to be 17,000) ) to increase data capacity (remember that higher spindle speeds meant less space?) as well as give them something that allowed them to decrease the heat and noise of the drive.
That's just about it.
Just remember, you can easily shrink a chip to decrease the distance in-between two points and make it run faster. Lower voltages, less power, faster speeds. But increasing the areal density, transfer rate, and seek time of a hard drive is significantly harder. Anything mechanical, as the others have, have inherent tolerances much lower than electrical devices. Just think about it this way. Hard drives have progressed significantly faster than cars have. Or optical technology. (CD/DVD ROMs and burners).
As a general rule of thumb, electronic devices increase an order of magnitude faster than mechanical devices.
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