The geometry of different hard disks varies, but always follows the same layout, no matter what the size or storage capacity. In each hard drive, each platter has 2 sides, and both of these sides are the same throughout each platter.

Tracks
Tracks are concentric circles of information on the disk. Unlike CD's, which has only one track that spirals out from the center, hard drives have many tracks. And unlike CD's, hard drive tracks start at the outer edges of the hard drive and work their way in. Tracks are read by placing the actuator arm and heads over the correct track, and the hard drive rotation will move the complete track under the head. There are many thousand tracks on each side of each platter in today's hard drives. Tracks are not referred to by themselves, instead they are part of cylinders, and broken down into sectors. The number of sectors in a track is dependent on which zone the track is part of.

Sectors
Are the smallest geometry that a hard drive has. Each sector is 512 bytes of user data, plus a few additional bytes are used for servo control, error detection and error correction. Sectors are sections of each track. Sectors are responsible for the storage of individual files.

Cylinders
Cylinders are grouping of the corresponding tracks on all of the platters. Because each platter's actuator arm is connected to the other actuator arms, all of the different platter's heads will always be on the same tracks. It is only logical that tracks be organized in cylinders. For example, track 3 on each platter compose the number 3 cylinder. This organization forms what can be visualized as an imaginary cylinder. Cylinders aren't needed for operation, but are used in all hard drives to make sector location easier.

Zones
Tracks are concentric circles on the platters, so the outer tracks will naturally have a larger circumference than the inner tracks. Because the outer tracks are bigger than the inner ones, they are able to hold more data. To handle this, a feature called zoned bit recording (ZBR) is used. ZBR is often referred to as multiple zone recordering or zone recording. Zoned bit recording is where outer tracks are grouped into different zones. These zones start on the outside tracks and are numbered in ascending order until it reaches the inner track.

Every track in a certain zone has the same number of sectors as every other track in that zone. Each zone can have a random number of tracks, and use a random number of sectors per track, it is up to the manufacturer to find the optimum numbers. Hard drive manufacturers can also add as many zones as they wish, with no set patterns to them because it is the internal control logic's responsibility for making zone conversions, so the feature is transparent to the rest of the computer.

Partitions
Partitions are when a drive is broken up into different portions. Each portion, or partition, has its own boot record and FAT. This allows a single hard drive to be partitioned to support different file structures.

Hard Disk Size		FAT16 Cluster Size	FAT32 Cluster Size
	32MB	2 KB (4 sectors/cluster)	-
	128MB	2 KB (4 sectors/cluster)	-
	256MB	4 KB (8 sectors/cluster)	-
	512MB	8 KB (16 sectors/cluster)	4 KB (8 sectors/cluster)
	1GB	16 KB (32 sectors/cluster)	4 KB (8 sectors/cluster)
	2GB	32 KB (64 sectors/cluster)	4 KB (8 sectors/cluster)
	7GB	-	4 KB (8 sectors/cluster)
	16GB	-	8 KB (16 sectors/cluster)
	32GB	-	16 KB (32 sectors/cluster)
	32GB - 2 TB	-	32 KB (64 sectors/cluster)

Fat12
An early FAT structure that used only 12 bits for cluster addressing. This was soon replaced by FAT16.

Fat16
Fat16 uses 16bit locations to reference files. This imposed some limitations. There could be only 2¹⁶ clusters, or exactly 65536 files. FAT16 used a dynamic size for each cluster depending on the hard disk capacity. Because of the 65536 limitation, FAT16 tried to get the most out of the hard drive. Larger hard drives would waste large portions of their space because even the smallest 1 byte file would consume an entire 32k cluster. Fat16 was limited to files only 2 GB's or less. Hard drives over 2 GB's would have to be partitioned into multiple drives to overcome this limitation.

Fat32 and VFAT
Fat32 uses 32bit locations to reference files. It was first introduced with Windows95and it allows for addressing of 4294967296 clusters, a yields a storage capacity of 2 terabytes. VFAT is short for Virtual File Allocation Table, which is another name for FAT32.

New Technology File System
NTFS is used exclusively with Windows NT and Windows NT based Operating Systems. It offers 32-bit file storage like VFAT, but offers data encryption and cluster compression.

High Performance File System
HPFS first appeared is IBM's OS/2 Operating system. It is no longer used by any other OS, but it did feature support for 2TB disks and filenames up to 256 characters.

Construction | Storage & Operation | Geometry & Partitions | Data Encoding & Floppy Drives