Areal density is often used as a technology growth-rate indicator for the hard disk drive industry.
is defined as the product of the linear bits per inch (bpi), measured along the length of the tracks around the disk, multiplied by the number of tracks per inch (tpi), measured radially on the disk (see Figure 9.1). The results are
Figure 9.1. Areal density, combining tracks per inch and bits per inch.
Drives record data in tracks, which are circular bands of data on the disk. Each track is divided into sectors. Figure 9.2 shows an actual floppy disk sprayed with magnetic developer (powdered iron) such that an image of the actual tracks and sectors can be clearly seen. The disk shown is a 5.25-inch 360KB floppy, which has 40 tracks per side, and each track is divided into nine sectors. Note that each sector is delineated by gaps in the recording, which precede and follow the track and sector headers (where ID and address information resides). You can clearly see the triple gap
Figure 9.2. A 360KB floppy disk media sprayed with magnetic developer (powdered iron) showing the actual track and sector images.
Notice that sector 9 is longer than the rest; this is to enable rotational speed differences between drives so that all the data can be written before running into the start of the track. Also notice that a good portion of the disk surface isn't used because it is simply
Areal density has been
At the current growth rate, within the next year or so, drive manufacturers will achieve areal densities of approximately 100Gbit/sq. in., which is considered near the point at which the superparamagnetic effect takes place. This is an effect in which the magnetic domains become so small that they are intrinsically unstable at room temperature. Techniques such as extremely high coercivity media and vertical polarity recording are
Figure 9.3 shows how areal density has increased from when magnetic storage was first developed (1956 RAMAC) through the present time.
Figure 9.3. Evolution of areal density in magnetic disk storage.
To increase areal density while maintaining the same external drive form factors, drive manufacturers have developed media and head technologies to support these higher areal densities, such as ceramic/glass platters, GMR (giant magneto-resistive) heads, pseudo-contact recording, and PRML (partial response maximum
To fit more data on a platter of a given size, the tracks must be placed closer together, and the heads must be capable of achieving greater precision in their placements over the tracks. This also means that as hard disk capacities increase, heads must float ever closer to the disk surface during operation. The gap between the head and disk is as close as 10 nanometers (0.01 microns) in some drives, which is approximately the
The one thing that has always been the cornerstone of the PC industry is
. With disk drives, this is evident in the physical and electrical form factors that comprise modern drives. By using
Over the years, disk drives have been introduced in several industry-standard form factors, normally identified by the approximate size of the platters contained inside the drive. Table 9.1 lists the different disk drive form factors that have been used in PCs and portables.
Table 9.1. Hard Disk Form Factors
Currently, 3.5-inch drives are the most popular for desktop and some portable systems, whereas the 2.5-inch and smaller drives are popular in laptop and other portable systems.
Shugart Associates first introduced the 5.25-inch form factor along with the first 5.25-inch floppy drive back in 1976. The story goes that Founder Al Shugart then left that company and founded Seagate Technologies, which introduced the first 5.25-inch (Model ST-506, 5MB capacity) hard disk in 1980, predating the IBM PC. IBM later used the Seagate ST-412 (10MB) drive in some of its PC-XT models, which were among the very first PCs to be sold with hard drives built in. The physical format of the 5.25-inch hard disk back then was the same as the 5.25-inch full-height floppy drive, so both would fit the same size bay in a chassis. For example, the original IBM PC and XT models had two 5.25-inch full-height bays that could accept these drives. The first portable systems (such as the original Compaq Portable) used these drives as well. Later, the 5.25-inch form factor was reduced in height by one-half when the appropriately named 5.25-inch half-height floppy drives and hard drives were introduced. This allowed two drives to fit in a bay originally designed for one. The 5.25-inch half-height form factor is still used as the form factor for modern desktop CD-ROM and DVD drives, and is the standard form factor for the larger drive bays in all modern desktop PC chassis. Early portable PCs (such as the IBM Portable PC) used this form factor as well.
Sony introduced the first 3.5-inch floppy drive in 1981, which used a smaller width and depth but the same height as the half-height 5.25-inch form factor. These were called 3.5-inch half-height drives , even though there was no such thing as a "full-height" 3.5-inch drive. Rodime followed with the first 3.5-inch half-height hard disk in 1983. Later 3.5-inch floppy and hard drives would be reduced in height to only 1 inch, which was just under one-third of the original 5.25-inch full-height form factor (these were sometimes called 1/3-height drives ). Today, the 1-inch-high version has become the modern industry standard 3.5-inch form factor.
PrairieTek introduced the 2.5-inch form factor in 1988, which proved to be ideal for laptop/notebook computers. As laptop sales grew, so did sales of the 2.5-inch drives. Although PrairieTek was the first with that form factor, other drive manufacturers quickly capitalized on the market by also introducing 2.5-inch drives. Finally, in 1994 Conner Peripherals Inc. paid $18 million for PrairieTek's 2.5-inch disk drive technology, and PrairieTek went out of business. Since the 2.5-inch drives first appeared, virtually all laptop/notebook systems used them. Although 2.5-inch drives can also be used in desktop systems, the 3.5-inch drive continues to dominate the desktop market due to greater capacity and speed along with lower cost.
2.5-inch drives have been manufactured in various thicknesses (or heights), and many notebook or laptop systems are restricted as to how thick a drive they will support. Here are the common thicknesses that have been available:
By far the popular sizes are 9.5mm and 12.5mm, which are the sizes used by most laptop/notebook systems. Currently most drive manufacturers are
The 1.8-inch drive was first introduced by Integral Peripherals in 1991 and has had problems gaining acceptance in the
During 1998, IBM introduced a 1-inch drive called the MicroDrive, incorporating a single platter about the size of a quarter! Current versions of the MicroDrive can store up to 4GB or more. These drives are in the physical and electrical format of a Type II Compact Flash (CF) card, which means they can be used in almost any device that takes CF cards, including digital
HP introduced a 20MB 1.3-inch disk drive called the KittyHawk in 1992, originally intended for the handheld computer market. In 1994 HP followed with a 40MB model. These small drives were expensive and proved to be too far ahead of their time, as were the handheld computers they were intended for. After two years of low sales, HP discontinued the KittyHawk family in 1994.
Other Form Factor Issues
Laptop manufacturers have various ways of mounting hard drives in the system, which can cause installation or upgrade compatibility problems. Most systems use a caddy or some type of special bracketry to hold the drive and possibly to make the electrical connections to the system.
This makes the physical part of an upgrade as easy as inserting a new drive into the caddy and then mounting it in the system. You can purchase the drive with the caddy already installed, you can get a bare drive and swap it into your existing caddy, or you can get a bare drive and a new caddy separately. Usually it is much less expensive to get the bare drive ”whether you also add the caddy or not ”than it is to get one with the caddy
If you purchase a drive as a repair part from the system manufacturer, it will normally include the caddy or bracket with the drive (see Figure 9.4). However, you will usually pay more than double the price of a bare drive alone. If you purchase a bare drive from a third-party manufacturer, it will not include the caddy or bracket, unless you ask for this separately. Depending on the model, a caddy alone can sell for $50 or more. If you are merely swapping the new drive for the one that is in the system, you won't need an extra caddy and instead can simply swap the caddy or
Figure 9.4. Hard drive caddy used in Thinkpad 770 laptops.
Using a caddy can make swapping drives very easy. In many
For example, in the ThinkPad 770 series, you can remove the existing hard drive and install a different one in under 10 seconds! To remove the drive you merely move a slide switch to release the cover plate, grab the pull tab on the drive caddy, and slide the caddy (with the drive inside) out of the chassis. To install a new drive you slide the caddy (with the drive inside) into the bay, attach the cover plate, and slide a switch to lock it into place.
Unfortunately, not all systems are designed in this manner, and if you purchase a bare drive that did not come with the correct caddy for your system, it will take time to remove and reinstall the drive from the existing caddy. Note that the caddies and brackets are not very interchangeable; that is, they are normally unique to the make and model (or series) of system they are designed for.
If you wish to have several drives that can be quickly installed in a system, then having extra caddies or brackets can be
One important aspect of laptop hard drive installation or upgrades that you must be aware of is the drive support provided by the system's BIOS. The BIOS in some systems, and particularly older ones, might offer limited hard drive size options. In some cases, Flash BIOS upgrades, which provide support for additional drives, might be available for your system.
After BIOS support, you need to be concerned about the physical mounting. Whether you are going to use your existing caddy or get a new one (if required), you must be