Old Disc Drives | Information

Synopsis

A little technical information does not hurt. If you are not into electronics or technology, this section might seem boring.

Drive History

In the beginning, disk drives were huge, noisy, low-speed devices that were designed to communicate with a special controller connected to a personal computer or a mainframe. They quickly evolved and decreased in physical size and increased in capacity.

The picture below shows two of my all-time favorite disk drives. The Quantum ProDrive LP-52s 50 Mb disk drive which is very easy to remember because of its unusual capacity, and the Seagate ST-351A/X 40 Mb disk drive, which I consider the best of all the IDE stepper drives.

Other approaches to computer storage media include solid state drives. At some point in the late 1980s and early 1990s, there were some attempts to make them with special static RAM chips. Quantum used to manufacture them for a while, but they were very expensive, and they required another expensive piece of hardware: a SCSI host adapter. They didn't make it to ordinary people like me and you. However, conventional drives that include moving parts such as platters and heads, were quickly adopted and gained huge popularity, mainly due to the low cost per Mb and their enhanced reliability.

Later edit: As of 2011, solid state devices (SSD) have made a spectacular return in the form of NAND memory. This is new technology and is the future of computer data storage.

Even later edit: In 2021, all my computers and network-attached storage devices run SSDs. So my ten-year-old prediction was accurate.

Platters

You probably already know that a hard disk drive contains a number of one or more rigid aluminum or glass platters, the said discs. If it weren't for these discs, they wouldn't be called hard disk drives. These discs are perfectly machined in order to ensure accurate movement. They are coated with a high-quality magnetic material that has the property of switching states as the head writes individual 0 and 1 signals. Each individual platter has the same geometry that divides it into sectors and tracks.

In order to reach for the specific data you need, the read/write heads first need to be correctly positioned on the right track and then wait for the sector that contains your data to pass under them. This process is, among others, what defines the drive seek speed.

The platters are coated with an oxide material, most commonly found on pre-1990 drives, or a thin film (well, technically still a magnetic material), commonly found on all other drives.

Actuator Assembly

Disk drive units read data from the platters by using an array of heads mounted on a mobile actuator. The heads are mechanically connected to the actuator via flexible stainless steel spring tensioning arms that are calibrated to the required pressing force. Each platter has one or two corresponding heads, depending on the drive configuration and factory design. Early drives used one platter for control and positioning purposes. Modern drives interleave positioning servo information with the actual data.

These heads are precision electronic devices containing small coils and very tiny gaps, all located on a special supporting material. The magnetic heads can be either traditional metal in gap (MIG), magnetoresistive (MR), giant magnetoresistive (GMR) or tunneling magnetoresistive (TMR).

During write operations, their role is to magnetize the particular portion of the platter(s), defined by track and sector, where the drive controller knows there is empty space available. The electric signals from the drive electronics are converted into magnetic fields that further magnetize the appropriate section on the platter surface.

During read operations, the heads detect the previously magnetized disc surfaces and convert the stored magnetic fields into electric currents.

The magnetic heads do not directly touch the platters. Instead, they fly over the surface of the platters at a very low distance, measured in only a few nanometers.

In the picture, I illustrate an array of heads mounted on the arm of the actuator of an old Seagate 5.25" full-height SCSI disk drive. Note the tiny individual purple cables that connect the individual heads to the drive electronics. The actuator assembly is a high-precision electro-mechanical part.

The array of heads is connected to a series of head amplifiers. These are electronic devices built as integrated circuits and their role is to amplify the logic signals that the drive electronics are sending to the heads, or those that are read by the heads from the platters. To ensure electric stability, head amplifiers also adapt the impedance of the signal line. The electric signals are transported to, and from, the head block to the drive electronics through a flexible ribbon cable.

Modern drives contain dedicated integrated circuits specially manufactured for them. In the picture you can see three individual head amplifier chips. You would think there is a single head amplifier per chip. But instead, each of these particular integrated circuits contains six different head amplifiers built-in. In our case, the drive has eight platters and sixteen heads. So the three individual head amplifier integrated circuits give us eighteen individual amplifiers. Naturally, two of them are unused. This is a very common practice.

The head amplifiers are always mounted as close as possible to the head block assembly. So it's very likely that you'll find them under the top cover, mounted on a flexible PCB that extends via flexible ribbon cable to almost half the length of the actuator assembly.

The actuator is moved by either a stepper motor or a voice coil. The stepper motor technology belongs to the 1980s and the early 1990s, while voice coil technology is what all modern drives use. In the picture you can see a very large and powerful voice coil that powers the movement of the sixteen-head actuator assembly that I was talking about above. Below the voice coil you can see a very strong neodymium magnet. I removed the top one for obvious photographic reasons.

The voice coil is... just a coil, built from enamel-insulated copper wire. It is called so because its operating principle is very similar to the coil that drives the movement of an audio speaker, basically a linear motor. The voice coil receives electric currents with a specific intensity calculated by the drive electronics in order to produce a precise movement of the actuator to the required track. These electric currents generate an electromagnetic field in the coil which forces it to move accordingly within the permanent magnetic field generated by the top and bottom neodymium magnets.

The voice coil servo positioning information is often read by the heads from one of the platters or from interleaved encoded position data. When power is applied to a disk drive unit, it first performs a calibration routine. That is what that crazy random start-up squealing sounds mean. The calibration process ensures the heads will always correctly track servo positioning data as well as regular data on the platters.

Older drives using servo motors, often send a train of pre-computed pulses to the stepper motor to move the head block to a specific track.

Drive Electronics

Each hard disk drive has at least some sort of drive electronics that handle the spindle motor control and the actuator positioning commands. The drive electronics are also responsible for converting and interpreting the signals that come from the drive controller or from the read/write heads.

These days, drive electronics are often very complex microcomputers that have a central processing unit (CPU), individual RAM, and a specific ROM (firmware). All these, in addition to the various spindle motor and head block control circuitry. Pictured is a fraction of an over-engineered drive electronics PCB from the same drive I exemplified before. It is powered by an Intel 80C188 CPU. Now hold on a second. There used to be computers based on 80C186 and 80C188 CPUs. You can see at least a dozen other very large-scale integrated (VLSI) circuits and an army of passive components.

As technology matured, specific integrated circuits and microcontrollers were built in order to reduce the number of electronic parts and thus production costs. Nowadays, it's quite common to see drive logic boards with only one big ULSI integrated circuit and a few other supporting parts.

Interfaces

Hard disk drives need a way to communicate with the computer. This specific piece of hardware that connects the drive to the computer is called a controller. The controller has a specific electrical interface. Interfaces are of different types, the oldest one being ST-506 from 1981.

The ST-506 interface and its flavors, such as the ST-412 and ST-412 RLL, were the first that allowed drives of the era to connect to the personal computer. These interfaces use either modified frequency modulation (MFM) or run length limited (RLL) technology in order to transmit and encode data on the platters.

Both MFM and RLL interfaces are electrically identical but RLL uses a different data transfer format. It squeezes the data onto the platters in order to increase the storage capacity by up to 50 %. Common practices back then were to format an MFM drive with an RLL controller to gain more storage space while assuming a lower storage media quality. There was also the possibility to format an RLL drive with an MFM controller to gain a more reliable storage medium but lose some storage space. These drives connect to the controller via two ribbon cables.

Integrated Drive Electronics (IDE) is the next big thing after the ST-506. The IDE interface means that the controller itself is located on the disk drive PCB instead of the separate extension logical board. This means that the IDE controller PC card is not really a controller but a host interface. The host interface translates the IDE commands and adapts them to the ISA or PCI bus of the computer. The drive is connected to the adapter via a single 40-pin or 80-pin ribbon cable.

As IDE technology matured, it evolved into ATA (AT Attachment), ATA-2, ATA-3 and so on. All these new technologies, built on the IDE basis, introduced new features such as ATA Identify Command, S.M.A.R.T. monitoring, and so on, while constantly increasing the communication bandwidth.

IDE was by far the most common drive interface throughout the 1990s and the early 2000s.

Later edit: As of 2011, the dominant technology is SATA-3, while IDE is long on its way to extinction, becoming obsolete.

Enhanced Small Disk Interface (ESDI) was also a successor to the ST-506 (ST-412) interface. However, this technology died as soon as IDE and SCSI drives progressed, and we have never heard of it ever since. ESDI interfaced the drive to the controller via two ribbon cables. There was also a Microchannel ESDI architecture, and your guess is correct, for the IBM PS/2 early systems. These drives connected to the MCA bus through a single card edge connector.

SCSI stands for Small Computer System Interface. It was and still is the ultimate high-speed communication technology available to connect disk drives to a computer. Back then, top-of-the-line systems, such as the Apple Macintosh or IBM PS/2, almost exclusively used a SCSI bus. The physical connection between the drive and the host adapter is done through a single 50-pin or 68-pin ribbon cable.

The last incarnation of SCSI is in the form of SAS, which stands for Serial Attached SCSI. The SAS interface is physically and electrically based on the SATA interface. But it uses the SCSI command set. This is mostly found in rack-mounted servers.

Last, but not least, there is the SATA interface, which uses the ATA command set over a serial bus. Despite being announced in 2000, as of 2025 it is still the relevant storage technology for the general consumer market. Almost all 2.5" and 3.5" magnetic and solid-state hard disk drives these days are SATA. Virtually, if it's not SAS, it's SATA.

Thank you for spending your time reading these old pages.