Photo courtesy of Seagate
Technology, Inc.The inside of a Hard Drive may appear as this (Seagate drive):
Photo courtesy of Seagate
Technology, Inc.
Notice the disk, called a "platter" or a surface. There are
multiple surfaces and platters in some hard disk drives.
The Read/Write head is on the robot arm that extends over the surface. The motor and controls for the read/write are in the bottom left corner, and the setup circuitry is in the bottom right corner. The container is open in this illustration; however, it is normally hermetically sealed to keep out dust.
The platters on a hard disk drive are metal, coated with some magnetic film material that can record data in the form of magnetized spots on the surface. The disk surface has "tracks" that are concentric circles (complete circles) that are next to each other on each surface. The tracks on the outside are larger than the tracks on the inner part of the surface. There may be 200 or more tracks per surface. If I have 10 surfaces, then I can have one cylinder composed of track 1 on each surface. Track 2 on all surfaces is one more cylinder. And so on, so that I have a cylinder for each track on the first surface.
Then each surface is subdivided into "sectors".
Each sector on each surface will be able to contain a specific amount of
bytes (8 bit characters), usually 512 bytes per each section of a track
per sector. Each sector contains a portion of each track on the surface. So,
if there are 200 tracks on the surface, a sector contains portions of 200 tracks.
The size of the sector determines the amount of data that can be written, and the amount that will be wasted if only a few characters are in a record. A one byte record written to a sector occupies the entire track in that sector.
A hard disk (and a floppy) must have a record that defines the disk to the CPU for access/writing. In a DOS environment, this record is the File Allocation Table (FAT). It contains the information about where to find anything written on the disk. Windows NT uses either DOS FAT or NTFS file systems. OS/2 uses a HPFS (high performance file system). The formats are different than FAT systems, but the information on the disk can still be found.
The disk rotates at about 3600 RPM in most older drives. Some of the newer drives rotate at 4500 to 7200 RMP and a faster 10,000RMP model was introduced recently. The rotation speed is a factor in the access speed of the drive. The read/write head must move to the proper track before it can read. Once at the proper track, it must wait for the proper sector to rotate under the head to read the data.
The factors that influence a hard drives performance are generally:
RAID Devices are multiple hard drives arranged in a cabinet, usually in a "tower" type cabinet. The thing that make them different from a large hard drive is primarily the multiple drives, which are 'hot-swapable'. This means that if a single drive of the RAID array fails, you can insert a new drive and the reconfigure to recover the data that was on the drive that failed.
The statement above is incomplete because there are also more things about RAID drives that make them different from normal hard drives.