Your computer's central processing unit (CPU) spends a lot of time waiting for your much slower hard drive to catch up. While operations inside the CPU are timed in nanoseconds (one billionth of a second), your hard drive operations are limited to mere milliseconds. Although considered very fast by human standards, a millisecond is actually 1,000,000 (one million) times slower than a nanosecond. Comparing the speed of your CPU to your hard drive is like comparing a Ferrari to a turtle.

So when you're loading your favorite word processor, for example, your CPU will have to wait until your hard drive can access all of the necessary data and load it into RAM (which also operates in nanoseconds), before it can proceed and give you a blinking cursor, at which point you're ready to type away.

Inside

A hard drive is made up of several unique components. The most popular of these components are the platters and the read/write heads. The platters are disc-shaped, usually made out of aluminum or glass. Unlike the flexible media in floppy diskettes, these platters cannot bend of flex, which is where the term hard disk comes into play. Another popular term to describe a hard drive is fixed disk, which is derived from the fact that the platters are fixed firmly in place and are not removable.

Physically, the operation of a hard drive is similar to a floppy diskette drive: the discs spin while the heads move over them to store or read data in tracks and sectors. Hard drives contain multiple platters with two sides each, where data is stored. For every platter, there is a read/write head, all of which move across the discs at the same time since they are all connected to one arm.

Although it is these heads that are responsible for writing data to the discs, they will never actually come into physical contact with the platters. These heads are kept suspended by an extremely thin cushion of air, and thereby float just a fraction of a millimeter above or below the platter. If dust or other small particles ever get into this part of the drive, it would easily be enough to cause the head to come into contact with the platter and do physical damage to it. Also, if a sufficiently large shock were to be applied to the drive while under power, the force could be enough to cause the heads to go crashing into the spinning platters and once again causing physical damage to the drive, not to mention data loss. This is know as a "head crash."

Performance

The average seek time of the drive is the amount of time it takes for the heads to move from one cylinder on the drive to any other cylinder.

The average access time, takes into account the drive's latency. Latency is defined as the average time (in milliseconds) that it takes for a sector to be available after the heads have reached a track. This figure is generally half of the time it would take for the disk to fully rotate once. On a drive with a spin rate of 4,500 RPM, a full rotation would take 13.33 ms, thereby making your latency equal to 6.67 ms. The average access time of the drive is the sum of its average seek time and latency. This gives us the average amount of time that is needed before a sector (chosen at random) can be accessed.

The maximum transfer rate

Transfer rates come in two different flavors: a disk transfer rate, and a host transfer rate.

• The disk transfer rate represent the speed at which data is moved to and from the drive. This is the number most often reported by hard-drive benchmarking software because it shows you the actual number of kilobytes that can be transferred in any given period of time. The disk transfer rate of a drive depends on the spin rate of the drive. A drive with a spin rate of 5400 RPMs will outperform a similar drive at 4500 RPMs.
• The host transfer rate represents the speed at which the CPU can access information from the controller. It's safe to say that all drives and system BIOSes manufactured as of last year will have support for PIO (Processor Input/Output) Mode 3 and Mode 4. The maximum theoretical transfer rate for PIO Mode 3 is 11.1 MB/sec. and 16.6 MB/sec. for PIO Mode 4.

A combination of a low average access time and high spin rate
is a sign of a drive that will deliver maximum performance.

Partitions

Every file you store on a hard drive relies on some kind of file addressing system to organize all your data. Among PC's, the most common file system is known as FAT (File Allocation System). Windows 95 uses a modified version of FAT called VFAT (virtual FAT), which adds support for long filenames. Windows NT supports both FAT, and its own NTFS format. NTFS is the most advanced file system. It allows access to partitions up to 16 PB (petabyte) in size. [ Equal 16 million terabytes - 16 followed by 18 zeroes).] Each file system must organize data into segments called clusters. Under the FAT file system, the maximum size of any one cluster is 32KB (32,768 bytes). The maximum number of these clusters is limited to 65,536. When you multiply these two numbers, you come up with what has come to be termed the "2.1GB barrier," referring to the maximum number of bytes that can be allocated on the drive.

Keep in mind that every file on your hard drive must occupy at least one cluster. This means that if you had a tiny batch file which was 156 bytes long, the FAT system would automatically allocate an entire cluster to that file, despite the fact that it's so small. What a waste of space.

Microsoft later created FAT32 to support partitions larger than two gigabytes and pathnames greater that 256 characters. It also allows more efficient use of disk space since clusters are four kilobytes rather than 32 kilobytes.