Part 2: Disk Media Technology

Media really means material. The inside of a hard drive contains one or more platters or plates that are used to hold the magnetic information. These platters consist of a substrate layer and several thin layers of material designed to hold the data and protect it.

Each platter has its own set of read/write heads. Sometimes, there is only one read/write head for a given platter but usually there are 2. Each head sits on a metallic arm that functions as a spring and extension. The spring acts to keep the heads as close to the platters as possible. Head technology may be addressed in a future post.

The platters start with a "substrate" layer such as aluminum or glass. The substrate layer is actually the support for the other materials. The substrate is polished to a flat surface to keep it as smooth as possible. The less defects here the more data you can pack in. Additional smoothing may be accomplished by adding a "thin film" of substrate. Once the surface is prepared, additional "thin film" layers are applied using either electroplating methods (cheaper but almost phased out) or "sputtering" methods. These layers vary in thickness from an amazing 1 nanometer (1 billionth of a meter) to 30 nanometers.

The first layer is the magnetic layer. It was originally iron oxide paint. Rumor has it that the first platters used the same paint used on the Golden Gate Bridge. Modern drives use a cobalt iron mix to create a harder layer with better magnetic characteristics. So, it has better durability and is able to hold much more data. There are actually 3 "sub-layers" to create the magnetic layer. There are 2 layers of magnetic material with a layer of the element Ruthenium in between, creating a super magnetic sandwich. The 2 magnetic layers act to reinforce each other's magnetism.

The second layer is a hardening layer. It is usually a sputtered carbon layer. While we mainly think of carbon as soft, such as graphite in lead pencils or coal, diamonds are also made of carbon. When sputtered on in a thin film, most of the carbon is deposited as an amorphous solid (like coal) but a portion of the carbon crystallizes and acquires the hardness of diamond. This layer can be from 3 nanometers to several nanometers in thickness.

The last layer is the lubricating layer. Lubricants used are chemicals with names like z-dol and Z-tetraol. They form a regular, smooth and very thin layer on the surface of the carbon. These layers can be little as 1 nanometer thick. Unlike standard oil based lubricants these synthetics have unique properties. Besides thinness, they are more durable and don't evaporate like oils. However, they are sensitive to heat. Heat can cause the lubricant to break down or evaporate. Loss of this layer is a major factor in media damage.

Looking ahead to the future of disks the latest news is that there may still be as much as 5 times the density available for data. A new technology called, exchange coupled composite, which is made from alternating layers of fast changing and slow changing magnetic materials. The 2 layers act to dampen each other eliminating errors at such small distances. Another interesting approach being investigated is "bit patterned media" which uses etching of the media to create discreet pockets to hold the data.

Next I will try to address some of the causes and characteristics of hard disk failure.

Peter

Part 1: Introduction to hard disk technology - Historical Perspectives

Hard disk drives revolutionized the world. They are used in a myriad of electronic devices from computers to cell phones. The first hard disk was invented at IBM's research lab in San Jose, CA. It held 50 24" platters with a single set of movable heads. Its maximum capacity was 5 MB, a ridiculously small amount by today's standards. It was released in 1956 as the RAMAC (which stands for Random Access Method of Accounting and Control) and sold with one of their early computers, the system 305.

It wasn't until several years later and many technical innovations that the more familiar looking hermetically sealed hard drive was developed. It combined different technologies from several leading companies including: IBM, Shugart Ass., Telex, Tandon, Memorex and finally, Seagate Technologies. Seagate was founded by Alan Shugart and Fenis Connor. The first drives came in 5 MB, 8 MB and 10 MB sizes. I owned several of these early drives in the beginning of my computer consulting days. They contained a 5 1/4 " platter in a form factor about the size of the modern CD/DVD drives in your computer but as much as double the height. It wasn't long after that the smaller 3.5" disk drives were released. Hard Disk Drive capacity increased at an unbelievable pace. The smallest drive commercially available utilizes a 1" platter and is used in cameras and cellphones.

The early drives used a "stepper" motor with a steel band to move the heads. Track positioning was a simple count of the number of steps the motor made. Eventually, the stepper motor was replaced by the "voice coil" motorized head armature. Voice coil motors come from loudspeakers. It consists of a coil of wire used to create a magnetic field on demand and a strong magnetic. When electricity is applied to the coil it causes the coil to repel or attract the coil from the magnet. Voice coils were cheaper to manufacture and easier to control with less moving parts. While greatly improved, this same technology is still in use today.

A discussion of changes in disk technology, disk controller technology and their effects on performance and capacity will be in a follow up post.

Peter Dinhofer

Welcome

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Peter Dinhofer
President
Data Recovery Service, Inc.