A hard disk (also called a "hard drive") is much like a filing cabinet. The programs and data are stored on the hard disk and the computer accesses them as needed. When the computer accesses the hard drive, it is reading the stored information into memory. That memory is the temporary workspace. The original file is still on the hard disk and is left undisturbed. When the computer stores information, it writes the data to the hard disk. That process results in the old file being replaced or modified with the new information. If you save data to a new file, or install new software, the information is written to the disk in an available, unused portion of the disk.
Most hard disks are installed internally to the PC, although external models are available. Also, a hard drive can be installed using a modular bay to make the hard disk portable.
The hard disk contains a recording media onto which data is written magnetically. That media is the platter and it spins at high speed. Hovering just over the surface of the platter are the "heads". Those heads read and write data to the platter. When you hear a whining sound, that is the drive spinning up to full speed. When you here a clicking sound those are the heads moving to read or write data.
Older versions of Windows 95 cannot address more than 2GB on a drive. Therefore, it is necessary to partition larger drives into multiple smaller drives so that Windows can use the space. In later versions of Windows 95 and all subsequent upgrades (Win98, Me, 2000, XP), larger drives are not a problem.
The hard disk uses an interface to communicate to communicate with the PC. There are two basic interfaces in use, ATA (formerly called IDE) and SCSI (pronounced "scuzzy"). By far the most common interface is ATA. SCSI is a faster, more expensive and slightly more difficult to set up.
Other common terms that also apply to hard disks are EIDE, ATA, SATA, PATA, Ultra and UDMA. These all apply to ATA drives and reflect improved standards in speed and performance. As of this writing, Ultra DMA/133 is the latest standard and simply reflects the overall performance of the interface.
There are several measurements by which hard disks can be compared. First is their capacity, currently measured in gigabytes (GB). Next is the interface used, such as UDMA/66 or UDMA/100. Third is their RPMs, two common speeds are 5400 RPMs and 7200 RPMs, faster is better. Others include their seek time (the time it takes to find a specific piece of data), access time (the time it takes to begin retrieving the data) and transfer rate (how much data can be transferred per second.
Hard disk failures can be sudden and unexpected or they may give you some warning. Whenever you suspect that you may have a problem with your hard disk, IMMEDIATELY back up any critical files you have. You may back up data to floppies, a second hard disk, a CD burner, other computers on your network or even email files to yourself as attachments for later retrieval.
Hard disks are generally not repairable. When a drive fails you must replace it. Recovering data from a dead hard disk is possible. After you have installed a new hard disk, you may be able to access the "dead" drive as a second drive from which you can copy files. Otherwise, a file recovery specialist must be used. The cost for a specialist is high, often starting at $400 and possibly running into the thousands.