Spindle Rotation
As soon as the hard drive is powered on, it begins to rotate until powered down. This continuous rotation is necessary to keep seek times low. Constant rotation doesn't consume very much electrical current, but to power up, the spindle motor can draw over twice the normal current. This is needed to quickly accelerate the hard drive so that it can be used. This power surge may not be very much by itself, but when multiple hard disk devices are all powering on at the same time, a significant electrical current can be drawn. This is usually only a problem in server computers, which almost always use a SCSI interface. For this reason, SCSI drives can be programmed to delay power up individually, to prevent a current overload. Older hard drives had a rotational speed of 3600rpm, while most modern hard drives spin at either 5400rpm or 7200rpm with some high end hard drives rotating at up to 10000rpm and 15000rpm.

Power Management
Since hard drives constantly spin, a constant amount of energy is drawn. This spin can be unnecessary if not hard disk activity is present. Most Operating Systems have some form of power management, that will power down the hard drive after a defined amount of time of inactivity.

Head Parking
The read write heads have to be very close to the surface, and the best way to achieve this is ride over the surface. This means that they will occasionally touch the surface during operation, and while at rest. Hard drives were designed for this. When the platters are not spinning, the read write heads rest on the surface. When the platters start to spin, the heads will drag along the surface until the the surface is rotating fast enough for the heads to lift off on a cushion of air. When the hard drive powers down, and the platters stop spinning, and the reverse is done. The heads will slowly touch down onto the surface. The friction of the heads is small, but this friction will eventually wear down the platter's surface. To combat this wear, hard drives are designed with landing zones. The heads go through what is called head parking. While very old hard drives had to be manually parked by the operating system or BIOS, all newer hard drives will automatically park their heads even if power is abruptly lost. This is done on most drives through a spring which will pull the heads to the landing zone, but some drives have found different ways which are just as effective.

File Storage and Access

Areal Density
To increase storage capacity without increasing physical size, areal density is increased. Areal density is measured in bits per square inch (BPSI). Areal density is determined by multiplying the average number of bits per track by the average number of tracks per inch.

Increasing areal density not only increases the capacity of the disk, but also the speed in which data is read. At the same rotational speed, more bits will pass under the heads per second, increasing how fast data can be read.

Clusters
Data is stored in specific patterns on the platters. Each file is stored in one, or many cluster, and the location is stored at the beginning of the drive in the File Allocation Table. Individual files are not stored by sector locations, rather by cluster location. There are too many sectors to keep track of without performance taking a hit, so sectors are grouped into clusters by the operating system. A cluster is able to hold only 1 file at a time, no matter how small the file is. If the file is too big to fit in one cluster, multiple clusters are used until enough space is made. Because files don't exactly fill each cluster, the cluster's extra space is left blank and goes unused because a cluster can only be part of one file at a time. To prevent excess wasted space, clusters are made as small as possible, without causing a slowdown in performance.

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.

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