Hard drives explained

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Computer hard drives explained


In this article, we’ll take a fairly basic look at hard drives, what they are and how they work. In the early 1970’s, programmers were required to actually place data on the hard drive manually, and were able to judge their handy work by observing the hard drive head’s movement over the disk platters through a clear plastic cover. If the heads moved violently back and forth, data was not placed efficiently throughout the hard drive. If the hard drive heads moved smoothly, the programmer succeeded.


Hard drive geometry


The basic hard disk geometry has not changed much throughout the years, but placing data is not as big of a concern in recent years. Hard drives contain actual disks, called platters. The number of platters within the drive varies, but drives usually contain more than 2 and spin in unison. The speed at which the platters spin is stated by the hard drive RPM (Revolutions Per Minute). Most newer hard drives offer at least 7200 RPM speed, meaning the drive platters will spin 7200 times per minute by the way of a central spindle.


To read data from the platter and write data to the platter, a hard drive contains read/write heads at the end of an arm that extends between the platters by the way of an actuator shaft. There is one read/write head each for each platter surface (top and bottom) to allow maximum data storage. Data is stored within the platters on sectors and tracks, and actual files are stored in clusters. A sector is always 512kb in size, and is thought of simply as a section of the platter. A sector can also mean a segment, or a triangular wedge, of the platter. A track forms a concentric circle around the hard drive platter. Tracks are on all platters and form cylinders down the platter.


Contrary to what you may think, data is not written from outside in, completely filling the first platter, then down. Data is written straight down to the last platter and follow cylinders (from the outer most cylinder inward). Data is written to the top cylinder of the platter, then moves down to the second platter and finds the corresponding track on it (located at the same place, equal distance from the spindle as the first platter’s cylinder). Once the cylinder is filled, the hard drive write head will move to the second track and begin writing downward to form another cylinder. If a hard drive contains 500 tracks, it also contains 500 cylinders. Each cylinder contains the same amount of space as the other cylinders within the hard drive, regardless of their physical positioning of the tracks on the hard drive platters.


Formatting a hard drive


Formatting a disk through DOS or any other program is easy, but few actually know what’s happening during this process. Low-level formatting is usually done at the manufacturer, which writes sector and track information on the disk. Because sector and track markings fail to follow a standard or pattern, home users should not attempt to low-level format, as permanent damage is imminent.


Most people are familiar with high-level formatting, which simply creates a new boot sector, file allocation table and root directory. The file allocation table is a table that details where each file on the hard drive is located. Actually, when you delete a file from your hard drive, the file is not actually cleared; only the file’s entry in the file allocation table is deleted, so the operating system will not see it and appropriately updated hard drive space can be determined.


Hard drive partitions


Although you may have a 10gig drive, you can break this 10gig drive into two separate drives, one 5gig and one 5gig (and other segmented sizes). Fdisk creates partitions very easily and can be accessed from within DOS (by typing fdisk at the C: prompt). To create another drive, like D: or E:, a separate extended partition must first be created. A partition is simply a division of that hard drive. The primary partition holds C:, and after creating an extended partition, logical drives (drives with letter assignments) can be created.


The partition table, located at the beginning of the drive within the master boot sector, contains partition information for the hard drive. When the computer boots, it will examine the partition table, find an active partition and execute the boot program within the active partition. If no active partition is determined, an error “No Active Partition” will be displayed. The user from within the fdisk utility determines active partitions.


IDE drives


Most hard drives available today use the IDE (Integrated Device Electronics) standards. Within all IDE hard drives lies a controller board, which has been programmed at the manufacturer to instruct the hard drive’s read/write heads, how they move and interpret the hard drive’s data. The controller is located on the hard drive’s circuit board that connects to the computer’s system board (or motherboard) with a 40-pin data cable. As you may have guessed, older system boards did not have IDE capabilities, so an adapter card was required to allow an IDE hard drive to interact correctly with the system board. The adapter card plugged directly into the system board, and the 40-pin data cable would plug into the card, similar to a PCI card.


Enhanced IDE (EIDE) drives


This new standard allows for more flexibility when upgrading and working with computer hardware components. EIDE allows for up to four IDE devices on the same system, including CD-ROMs, tape drives or other hard drives. Confusing the matter, different manufacturers took advantage of the new standard and dubbed them different names. When choosing an EIDE hard drive, be sure your computer’s motherboard can support the standard. If not, you can purchase an IDE adapter card, which will fit nicely into your PCI slot. My system, for example, consists of two EIDE hard drives (one 75gig and one 40gig). Because these drives are both EIDE and my system supports IDE, I have an adapter card installed into PCI slot 2 and everything is merry.


To conclude, I hope you’ve learned a few things about hard drives; you may look at hard drives now in a whole new way. Here are a few hard drive manufacturers:


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