Disk
Connection: SATA or PATA
A laptop has either Serial or Parallel ATA support for the disk. SATA is newer and faster, but it uses a bit more energy from the battery. Modern desktop mainboards have more SATA connectors that provide better performance and use a much simpler cable.
Size: 2.5 or 3.5 inch
Laptops use a 2.5" hard disk (or even smaller on some ultra-thin models). Desktops traditionally have used 3.5" models. Although the difference appears to be just one inch in width, once you expand this to three dimensions the smaller disk is much, much lighter. Smaller size means less capacity (the largest 2.5" disk has only 1/3 the data on the largest 3.5" disk) but with a lighter disk to rotate and less distance to move the arms, the smaller disk uses much less electric power.
Capacity: 250G, 500G, 750G, 1T, 1.5T bytes
Disks come in all capacities. You can certainly buy old 80G disks, but they don't make much sense today. It costs some money just to manufacture the disk and motor and electronics, and the cost difference between 80G and 320G is only a few dollars. Above 320G, however, the disk has to add a second platter and set of arms. There used to be a sweet spot in the middle of the capacity range, and disk makers charged a premium price for their very largest disks. Today, however, prices reflect the economies of scale and a 1.5 TB disk costs only 20% more than a 1 TB disk.
Laptop 2.5" disks have a maximum capacity of 500G, and smaller sizes are more common, which makes the cost per byte much higher.
Enterprise disks installed in server farms for corporate database systems have less capacity than desktop systems. Disks are mechanical systems whose performance is determined by a motor that rotates the disk and another motor that moves the disk arms across the surface. A desktop or laptop user makes casual use of his disk, and when an end user fills a disk with files they typically are large sequential files (pictures, music, movies). A corporate database has hundreds of thosands of small records (every order, product, part, customer, employee) that have to be randomly accessed. You might be able to store all the corporate data for a billion dollar company on a $120 1.5 TB desktop disk, but with only one set of relatively slow arms to access all that data performance would be terrible. So corporate data is stored on smaller disks with better performance. Faster rotation, faster arms, and capacities of 75 or 150 GB. It costs a lot more to store 1.5 TB on ten 150 GB disks or twenty 75 GB disks, especially when you pay twice as much per disk for faster rotation and arm movement. Spreading the I/O load across all the disks gives much better performance, and in business the improved productivity and satisfaction is worth much, much more than a modest increase in hardware costs.
Rotation and Seek
A laptop disk rotates at 5400 RPM. Desktop disks rotate at 7200 RPM. Enterprise disks for corporate servers rotate at 10,000 or 15,000 RPM.
The computer tells the disk to read a piece of data. First the disk has to move the arms to the location between the center and outside edge of the disk where that particular piece of data is located [the "seek"]. Then it has to wait for the disk to rotate until the data moves under the arm location and can be read ["rotational latency"]. So if a corporate disk rotates at twice the speed of a desktop system (and three times the speed of a laptop disk) then the rotational latency is half (or one third).
There is a comparable but less well publicized difference in the speed of the arm movement. If you use more powerful motors (and more electric power to drive them) then the seek can complete much faster. Corporate disks typically can complete a seek in half the time it takes for a desktop disk.
However, if you are reading a very large file sequentially off the disk, the data rate is mostly determined by the density of the data recording. Here desktop systems with the highest possible capacity have an advantage. So if you want to read through a single 12 GB file, the desktop system is actually fastest. However, if you want to read 1000 random purchase orders in a database, then the data is so small that density doesn't matter and the corporate disk is twice as fast in both seek and rotational latency.
Number of disks
A laptop has only room for one disk. A corporate server can have dozens of disks. The desktop user has a choice.
If you buy a computer at Wall-Mart or from Dell, for the advertized low price you will get a single disk. Customers know to look at the CPU speed and memory size. However, they will often concentrate on the disk capacity assuming that one 1.5 TB disk is as good as two 750 TB disks. While they do hold the same amount of data, there is one common type of use where two disks can do in a minute what one disk requires an hour to accomplish.
Desktop disks can transfer 80 to 100 megabytes per second, but only if the arm stays in one area and you read one file sequentially. If you copy one file from one place on disk to another (as happens when you transform an audio or video file by, for example, editing the commercials out of a TV program) then the disk arms have to jump from the location of the old input file to the location of the new output file. On average a desktop disk can reposition arms 50 to 70 times a second, which means 25 to 35 jumps back and forth between two files. The application can buffer or the system can cache large amounts of data in memory to overcome this problem, but you cannot assume that they will always be smart enough to fix the problem.
You decide to process a recorded TV program and initially tell the program to put the output file on the same disk as the input file. The program begins and says processing will take an hour. You stop it and change the output file to use a second disk. Processing completes in a minute. If you really want the output file to be on the other disk, you can take a second minute to copy the output file back to the first disk and still be 58 minutes better off than if you used just one disk for the processing.
You can spend twice as much on a CPU that gives you only 20% faster processing. Once you spend $80 to get 4GB of RAM memory, additional memory can only be used by specialized applications. However, when you buy a second disk then, for many disk intensive applications, you get far more than a 100% performance improvement. You can get a 5000% improvement in file to file copy operations by avoiding arm movement delay.
Trends
3.5" to 2.5" migration
Disk capacity is always increasing, but business users and many casual home users don't need 1.5 TB disks.
The 2.5" disk is smaller, lighter, uses less power, and is easier to upgrade to faster performance. It is better in every way except for capacity. If you don't need the capacity, then this smaller form factor has become attractive.
Corporate servers used to have 3.5" disks, but today they mostly use the smaller 2.5" disk. This is not a laptop disk. It is thicker, faster, louder, and it uses a lot more energy than laptop disks. It just uses a lot less energy than older 3.5" corporate disks.
We may see some desktop systems migrating from 3.5" to 2.5" disks if this technical trend continues.
Solid State
Disks are mechanical systems that have to rotate and move arms. Flash memory chips have no mechanical components. Flash memory comes in several speeds and price ranges. The least expensive flash memory can be used in USB drives that allow you to carry your data around with you. To even begin to compete with disk performance, vendors must use much more expensive types of flash.
A solid state disk (SSD) does better with random access because it has no arms to move or rotational latency. It does not, however, match up with modern desktop disks on price, or capacity, or the tranfer rate of large sequential files like MP3 audio or movie video. This profile matches the requirements of corporate database systems, so the SSD will begin to replace server disks before it becomes widespread on desktops.
Removable
Laptop users typically don't have an option to install a second disk, but they can attach an external disk. A USB disk may only provide half the transfer rate of an internal disk, but that is still much better than a factor of 50 times slower performance using one disk instead of two. If the laptop offers an eSATA connection option, then external disks can be as fast or faster than the internal disk.
For $35 you can buy an external device that connects to a desktop or laptop using eSATA or USB. You drop a SATA disk (2.5" or 3.5") into the device, turn it on, and the disk becomes available to your computer. Tell the system to dismount the disk, remove it, and plug a different hard disk in to access other data.
Corporate on the Desktop
If you want your operating system to load faster and perform better, you might consider a solid state (SSD) disk for lots and lots of money. For about half that price, you can get a Western Digital "Rapter" series disk with 10,000 RPM rotation and twice the seek speed as an ordinary desktop disk. The system disk gets most of the random I/O and cannot be easily spread across a lot of disk arms. If you are careful, it is a fixed size that can fit on a more expensive, faster device. Then you store your pictures, music, and movies on other conventional desktop disks.
Green
Some energy savings can also be found in "Green" models of the 3.5" disks. There are two tricks. One is to rotate the disk at 5400 RPM when it is lightly used and speed it up to 7200 RPM as load increases. Another is "intelligent seek" where the disk electronics calculates the lowest power that can be applied to move the disk arm into position before the location of the next unit of data rotates into position. Higher power moves the arm faster, but that serves no purpose if when the arm gets to the right location the data is halfway around the disk and the arm will have to wait 2 miliseconds for the half rotation. So instead the disk uses less power to move the arm more slowly, though still fast enough to be ready when the data arrives.