Is it correct that in a hard disk both surfaces of each disk are capable of storing data?

Question

I have read that in a hard disk both surfaces of each disk are capable of storing data except the top and bottom disk where only the inner surface is used. Is it correct if yes then why is are there read/write heads on top and bottom?

From CBSE Class X1 Computer Science textbook:

Hard disk: A hard disk consists of one or more circular disks called platters which are mounted on a common spindle. Each surface of a platter is coated with a magnetic material. Both surfaces of each disk are capable of storing data except the top and bottom disk where only the inner surface is used. The information is recorded on the surface of the rotating disk by magnetic read/write heads. These heads are joined to a common arm known as access arm. This arm moves over the surface of the rotating disk

Also you can see the same thing in this link.

Answer 1

The read/write heads in a hard drive are not cheap. If the outermost sides of the platters were not being used for data storage, then the manufacturers wouldn't waste the money adding read/write heads there (plus the associated wiring, the extra weight, etc). Unnecessary hardware would be one more thing that could possibly cause problems, but would give no benefit in return. If you see a read/write head, then that side of the platter is being used for something.

That being said, drive manufacturers don't always use both sides of every platter. For instance, they might take an existing drive model, remove the read/write head for one side of one of the platters, and market it as a lower-capacity drive. Many times, having two slightly different variations of a common design is cheaper than designing and manufacturing two completely different models of drive.

Your textbook most likely got its information from outdated sources. The manual for the IBM 1301 Storage System (one of the first commercially-available hard drives) mentions that "the top disk surface and bottom disk surface … are not used for data processing operations" (archived copy, see page 6). This product was released in 1961 and resembles a modern hard drive in the same way that a Model A Ford resembles a modern automobile. Hard drive technology has come a long way since then and continues to change extremely quickly. Any description of the implementation details of a computing device are destined to become obsolete quickly. Thankfully, that sentence can be ignored and the reader will still have a good general idea of how hard drives work.

Answer 2

I worked on every major disk drive manufacturer of HDD’s including our own at Burroughs for 11 yrs starting in ‘83

All 14” and 8” Head-Disk-Assemblies (HDA) used the top surface for servo feedback and all the others were data heads with the exception of our 207 14” HDA which used embedded servo bits between each sector for position error feedback, just like all modern 3.5 and under disk drives.

Early 5.25” Drives from NPL(Hitachi) and Seagate all used stepper motors and all surfaces. Then Maxtor, Atasi, Miniscribe, CDC, made them seek faster with small linear or rotary servo motors with dedicated surface (usually top) for position feedback. DMA and Syquest used Hard Disk cartridges and again, all surfaces were used.

There are no HDD’s that do not use all surfaces with heads.

The reference information looking at the size of head-arms looks like 1960’s technology.

The insides of the 80’s technology were very clean with 1 to 100 particles per cubic foot recirculating clean air thru HEPA filters. Older technology may have suffered from contaminants on the outer surfaces and to get around high defects may have excluded those but I am not aware that IBM, Memorex, Burroughs, Hitachi, Toshiba, Fujitsu, and a dozen other companies ever had any such configuration.

The only possible reason in the 60’s or early 70’s to not use the outer disk would be the dual servo surfaces were used to correct for tilt error in the spindle bearing to head actuator bearing vertical alignment, but I think the authors did not understand how HDA’s work. Because it does not reflect how any HDA’s running today work.

The author’s name and title might reveal more info, but it is contradictory at best and ancient at worst description of how HDD’s operate. My job titles are in my profile.

Very few people get the opportunity to really understand how HDD’s work and I consider them the most complex electo-magnetic-mechanical devices that have evolved over decades of R&D with fortunes spent in improving the technology. Now the aluminum cobalt doped oxide surfaces will be sputtered on smoother glass surfaces and the aerodynamic flying height has shrunk from 50 u” to 10u” to 1u” down towards wavelengths of light acting as an air bearing with 1 million hours MTBF.

Answer 3

Single platter drives exist. Different manufacturers have had single platter drives in multiple standards for a long time. These by necessity require the top or bottom side to be used for storage. I dont doubt that some manufacturers have at some point decided that the outermost sides of a stack of platters should not be used for whatever reason, but its not a universal truth.

This specific one is unique due to the form factor not because it is a single platter.