Capacity and access speed

Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy disk drive. Consequently, hard disk drives can store much more data than floppy disk drives and can access and transmit it faster.A typical desktop HDD might store between 120 GB and 2 TB of data (based on US market data[10]), rotate at 5,400 to 7,200 rpm and have a media transfer rate of 1 Gbit/s or higher[citation needed]. (1 GB = 109 B; 1 Gbit/s = 109 bit/s)As of January 2009[update], the highest capacity HDDs are 2 TB[11].The fastest “enterprise” HDDs spin at 10,000 or 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s.[12] and a sustained transfer rate up to 125 MBytes/second.[12] Drives running at 10,000 or 15,000 rpm use smaller platters to mitigate increased power requirements (due to air drag) and therefore generally have lower capacity than the highest capacity desktop drives.Mobile, i.e., laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have lower capacity. A typical mobile HDD spins at 5,400 rpm, with 7,200 rpm models available for a slight price premium. Because of the smaller disks, mobile HDDs generally have lower capacity than the highest capacity desktop drives.The exponential increases in disk space and data access speeds of HDDs have enabled the commercial viability of consumer products that require large storage capacities, such as digital video recorders and digital audio players.[13] In addition, the availability of vast amounts of cheap storage has made viable a variety of web-based services with extraordinary capacity requirements, such as free-of-charge web search, web archiving and video sharing (Google, Internet Archive, YouTube, etc.).The main way to decrease access time is to increase rotational speed, thus reducing rotational delay, while the main way to increase throughput and storage capacity is to increase areal density. Based on historic trends, analysts predict a future growth in HDD bit density (and therefore capacity) of about 40% per year.[14] Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.The first 3.5″ HDD marketed as able to store 1 TB was the Hitachi Deskstar 7K1000. It contains five platters at approximately 200 GB each, providing 935.5 GiB of usable space;[15] note the discrepancy between the its capacity in decimal units (1 TB = 1012 bytes) and binary units (1 TiB = 1024 GiB = 240 bytes). Hitachi has since been joined by Samsung (Samsung SpinPoint F1, which has 3 × 334 GB platters), Seagate and Western Digital in the 1 TB drive market.[16][17]As of December 2008, a single 3.5" platter is able to hold 500GB worth of data.[18]
Capacity measurementsA disassembled and labeled 1997 hard drive. All major components were placed on a mirror, which created the symmetrical reflections.Capacity of a hard disk drive is usually quoted in gigabytes and terabytes. Older HDDs quoted their smaller capacities in megabytes, some of the first drives for PCs being just 5 or 10 MB.The capacity of an HDD can be calculated by multiplying the number of cylinders by the number of heads by the number of sectors by the number of bytes/sector (most commonly 512). Drives with the ATA interface and a capacity of eight gigabytes or more behave as if they were structured into 16383 cylinders, 16 heads, and 63 sectors, for compatibility with older operating systems. Unlike in the 1980s, the cylinder, head, sector (C/H/S) counts reported to the CPU by a modern ATA drive are no longer actual physical parameters since the reported numbers are constrained by historic operating-system interfaces and with zone bit recording the actual number of sectors varies by zone. Disks with SCSI interface address each sector with a unique integer number; the operating system remains ignorant of their head or cylinder count.The old C/H/S scheme has been replaced by logical block addressing. In some cases, to try to "force-fit" the C/H/S scheme to large-capacity drives, the number of heads was given as 64, although no modern drive has anywhere near 32 platters.Hard disk drive manufacturers specify disk capacity using the SI prefixes mega-, giga- and tera-, and their abbreviations M, G and T. Byte is typically abbreviated B.Most operating-system tools report capacity using the same abbreviations but actually use binary prefixes. For instance, the prefix mega-, which normally means 106 (1,000,000), in the context of data storage can mean 220 (1,048,576), which is nearly 5% more. Similar usage has been applied to prefixes of greater magnitude. This results in a discrepancy between the disk manufacturer's stated capacity and the apparent capacity of the drive when examined through most operating-system tools. The difference becomes even more noticeable for a gigabyte (7%), and again for a terabyte (9%). For a petabyte there is a 11% difference between the SI (10005) and binary (10245) definitions. For example, Microsoft Windows reports disk capacity both in decimal-based units to 12 or more significant digits and with binary-based units to three significant digits. Thus a disk specified by a disk manufacturer as a 30 GB disk might have its capacity reported by Windows 2000 both as "30,065,098,568 bytes" and "28.0 GB". The disk manufacturer used the SI definition of "giga", 109 to arrive at 30 GB; however, because Microsoft Windows, Mac OS and some Linux distributions use "gigabyte" for 1,073,741,824 bytes (230 bytes), the operating system reports capacity of the disk drive as (only) 28.0 GB.