Hard Disk Drives

Physical Structure and Geometry

• Platters: The physical disks contained inside a hard disk drive.
• Heads: One side of a platter used to store data. A hard disk drive that has four platters can have eight heads. However, if one of these heads is not used for regular data storage, then the hard drive will be listed as having only seven heads.
• Read/Write Head: The electro-magnetic device on the end of an actuator arm used to write or read data from the disk head. Each disk head has its own actuator arm and read/write head.
• Tracks:A set of concentric circles running around the disk head. These tracks are paths where data is stored on the disk head. Each disk head can have hundreds of tracks.
• Cylinder: The set of all tracks of equal diameter on all the disk heads.
• Sectors: Segments of a track, often compared to the "pizza slices" or "pie wedges" of a disk. Sectors hold 512 bytes of data.
• Sectors per Track: The number of sectors on each track.
• Clusters:Combination of contiguous sectors into one logical unit by the File Allocation Table (FAT). The size of a cluster depends on the FAT software being used and is set when the drive is partitioned. Clusters are also known as "File Allocation Units".
• CHS: An abbreviation for "Cylinders, heads, Sectors/track". Refers to the overall geometry of the hard disk drive.
• Cylinders x Heads x Sectors/Track x 512bytes: Formula to calculate the data storage capacity of a hard disk drive in bytes.
• RLL: Run length limited. Type of encoding commonly used for storing data on hard disk drives. The encoding scheme is determined by the hard drive controller card.
• Modern drives use voice coil motors to move the actuator arms. This technology often requires the drive to have an extra disk head for navigation purposes, and also negates the need for "parking" modern disk drives.
• Access Time: The time it takes the hard disk drive to find a section of data on the disk drive. The access time is currently measured in milliseconds, with typical access speeds being under 10 milliseconds.
• Data Transfer Rate: The time it takes the hard disk drive to actually transfer data from the disk drive to the CPU. This is typically measured in megahertz or megabytes per second.

Drive Interfaces

• The original IBM hard drive used an interface called ST-506, which is now obsolete.
• The ESDI built on the ST-506 interface, but it too is now obsolete.
• Most personal computers use an EIDE (Enhanced Integrated Drive Electronics) interface. In common parlance, these are still referred to as IDE or ATA drives,which were earlier versions of the interface.
• IDE / EIDE drives use 40 pin cables.
• SCSI (Small Computer Systems Interface) is a drive interface as well as an expansion interface. SCSI comes in many different flavors and drives will usually have either a 50-pin cable connection or a 68-pin cable connection. SCSI devices are typically found on higher-end systems or servers.
• CD-ROMs, DVDs, Tape Drives, and various other storage devices will also use either the EIDE interface or SCSI interface. Before installing one of these devices, you must make sure that your system supports the correct interface.

EIDE Drive Installation

• EIDE drives must be connected via 40 pin cable to either a controller card (on older PCs) or a controller port on the motherboard. Computer systems typically have two controller ports, with one being labeled "Primary Controller" and the other being labeled "Secondary Controller". The main boot drive must be connected to the Primary Controller.
• An EIDE connector ribbon can connect up to two drives to the controller. The drive's position on the cable is irrelevant.
• Each drive must be set as either a "master" or "slave" (some systems also allow for "cable select", but this implementation is non-standard). If the cable has only one drive, it should be set to "master".
• When installing a second drive on a system, it may be installed as either the slave on the primary controller, or as master on the secondary controller.
• The colored stripe on the 40-pin cable should be connected to pin number one on the drive. If the pin number cannot be indentified, it is traditionally the pin closest to the power supply. If connected incorrectly, the system will not recognize the drive and may not even boot, but no hardware will be damaged.
• After installation, a hard disk drive will need to be configured in CMOS, unless the CMOS is set to "auto-detect" the type of drive. Press the "magic button" (typically [DEL] or [F1]) to enter the CMOS setup and either enter the drive geometry information or use the "auto" setting on most modern CMOS.
• LBA (Logical Block Addressing) and ECHS (Enhanced CHS) are two technologies designed to lie to the BIOS so that the system can access drives larger than 504MB. These technologies permit use of drives up to 8.4GB. In CMOS, ECHS is enabled using the "Large" mode, and LBA is enabled using "LBA" mode.
• INT13 (Interrupt 13) extensions permit systems to access drives larger than 8.4GB. With INT13 support, systems can access drives up to 137GB.
• The steps to installing a new hard drive are as follows:
  1. Physically connect drive
  2. Configure drive in CMOS setup
  3. Run FDISK to set partitions on drive
  4. Format the drive

Partitions, Formats, and the File Allocation Table

• FDISK is the program used to partition a hard disk drive. Partitioning allows one single drive to be treated as if it were two or more separate drives. Even if a drive is not going to be "partitioned", you must use FDISK on a new disk to create the primary partition and set it as active.
• Drives are usually partitioned either to create a dual-boot system (two different operating systems), or to bypass disk drive size restrictions associated with an operating system.
• Disk drives must be formatted before they can be used. Formatting creates the File Allocation Table and the root directory.
• FAT (File Allocation Table) is a system database that records the status of each cluster on the drive, including which clusters hold which files. In essence, it is the main map of the hard drive.
• A file that is fragmented has portions stored in non-contiguous clusters. Fragmented files take longer to access and retrieve, thus programs like Disk Defragmenter are used to keep file segments together.
• When a file is erased, the FAT replaces the first letter of the file name with the Greek letter phi ; all other information about the file remains intact until the file is over-written. Unerase programs are able to restore "erased" files by simply replacing the phi with the original letter and thus restoring the files active status in the FAT.

Troubleshooting

• Reading errors on a hard disk drive can often be fixed using ScanDisk or a similar utility. These errors occur when a cluster on a drive "goes bad". The ScanDisk utility attempts to recover the data in the bad cluster, then marks the cluster as bad so that it will not be used in the future.
• If a hard disk utility finds errors on a drive, then finds more errors when run soon afterward, the drive is dying. Replace it immediately.
• Lost CMOS information, poor cable connections, and bad boot sectors are all common causes of hard drive errors or "failures". When a failure occurs, check the CMOS setup, all cable connections, and then use a disk utility to try to recover the drive.
• Experience has shown that when a drive makes horrible clunking noises, it's dead.
• All hard drives die eventually. Back up frequently to make the inevitable restoration less painful.
• Hard disk drives do come with warranties. If a drive has died, use your drives stock number to see if the drive is still under warranty