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.

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    \$\begingroup\$ I've lived long enough to see just about everything. That includes drives with both top and bottom surfaces used for data, or just one side and not the other. And some use all the surfaces, including the outer surfaces of the outer platters. (Obviously, a single platter disk would be using at least one outer surface, by definition, and single platter disks abounded in the earlier days of my experience.) Also, some surfaces provide tracking guidance, too, where mechanical means aren't used for that purpose. So I cannot say "yes" and I cannot say "no." It's not a yes/no question. Not generally. \$\endgroup\$
    – jonk
    Apr 27, 2021 at 19:21
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    \$\begingroup\$ I don't think the source you cited is authoritative. Do not read it as any kind of official, standard definition. Assume it is just one possible description for a multi-platter hard drive. \$\endgroup\$ Apr 27, 2021 at 19:44
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    \$\begingroup\$ @jonk - Hi :-) Can I encourage you to convert that comment into an answer, please? It doesn't fit the uses for a comment (e.g. it doesn't ask for clarification, or tell the OP how to improve the question). Importantly, the OP can't "accept" it. Having an "answer-in-a-comment" can discourage other people from writing an answer, so we sometimes end up with a question that gets deleted by an OP (and then your comment/answer would be lost - I see this happen often) or perhaps it survives, but with no accepted answer. Thoughts? \$\endgroup\$
    – SamGibson
    Apr 27, 2021 at 20:10
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    \$\begingroup\$ @SamGibson Well, it implies that the question could be improved by specifying the drive device that includes heads on the outer surfaces. Then one could answer by simply saying that the specified drive isn't covered by the specified text. Or it could be improved by not asking a 'yes/no' question -- though I'm not at all able to reach into the OP's mind to guess at what question would lurk in there, knowing it's not a yes/no question. And I've no interest in writing a long history of what I've ever seen done, either. (I worked under Philips on gallium ion beam milling for disk heads.) \$\endgroup\$
    – jonk
    Apr 27, 2021 at 20:23
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    \$\begingroup\$ @jonk - Thanks for your reply. I respectfully disagree about the premise of your reply (i.e. that the original comment is allowed as a comment, as it implies constructive criticism for the OP - IMHO it does much more than that), but I'm not going to do more at this stage. Again, and with sadness at my lack of persuasive skills, thanks :-( \$\endgroup\$
    – SamGibson
    Apr 27, 2021 at 20:43

3 Answers 3


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.

  • \$\begingroup\$ Although this answer is logical, it is factually incorrect on the capacity head shortage per platter .The outmost layers "might have been" avoided in the 1st year of production on ancient 14" models from some supplier , who shall go nameless, who could not control the particle shedding rate with their designs with their HEPA filter and housing under 100 ppm/cuft. That did not include Memorex or Burroughs where I worked. There was however 1 surface dedicated for Servo control which is not embedded between every sector on every track of every surface. Nobody ships a HDD platter without heads. \$\endgroup\$ Oct 27, 2021 at 21:34
  • \$\begingroup\$ The servo layer was always in the middle in order to minimize offtrack from tower actuator tilt error. Out layers were contaminated the most from particle shedding from the casting with turbulence of 100 mph air flow. I have no idea why this answer was accepted based on facts. \$\endgroup\$ Oct 27, 2021 at 21:39
  • \$\begingroup\$ @TonyStewartEE75 I've taken apart a lot of drives over the years (harvesting the magnets) and I've encountered several drive models that had fewer heads than platter surfaces. These were mostly low-end consumer drives from the 1995-2005 time frame. Definitely not common, but not unheard of. \$\endgroup\$
    – bta
    Oct 28, 2021 at 2:51
  • \$\begingroup\$ That was around the time-frame when conventional recording was pushed to the limit and the end of that era was the start of vertical recording. It wouldn't surprise me that missing heads were on the upper/lower surface for the above reason I stated, But then this was long after I got out of the biz. Otherwise if that wasn't true and entire platters were headless, you could be right. I was reading if a 777 was lying at the same relative geometry above the earth's surface it would be going at Mach 800. HDD's are the most complex simple looking bricks I've ever worked on. \$\endgroup\$ Oct 28, 2021 at 7:22

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.

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    \$\begingroup\$ You remind me my 1980 days, when I heard the saying: "Happiness is having your own disk!". I bought my first hard disk, a Seagate 40MB, with HK$4,000 (about US$500) and my colleagues, who couldn't afforded one, became to respect me more than I deserved. Fast backward only some 20 years, I still owned a couple of 250MB 3M mag tapes, which shorted sighted me thought were my "permanent", "mass" storage. Oh well, these days I would lose face if the tick-tok gen guys discovered what I am owning now! :( \$\endgroup\$
    – tlfong01
    Apr 28, 2021 at 3:45
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    \$\begingroup\$ (1) You reminded me the first time I surprisingly heard that the read/write heads do not touch the disk surface but surf on a thin air layer, sort of air bearing. (At that time I owned casette tapes with heads touching the tape surfaces.) (2) I also surprisingly heard from my bad EE enggr friends, that when I bought a 40MB HDD then, the 40MB HDD actually has more platters to store perhaps 120MB. But the evil manufacturers disabled some platters to make the capacity look smaller, so the poor hobbyists like me could afford, and we didn't need to use that 40MB big storage anyway. \$\endgroup\$
    – tlfong01
    Apr 28, 2021 at 3:56
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    \$\begingroup\$ Burrougs Glenrothes (1980) here ... the 20/80MB 14 inch disk was the 211 then... Can't remember the 10MB floppy drive model no. \$\endgroup\$
    – user16324
    Apr 28, 2021 at 9:25
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    \$\begingroup\$ For kicks on the Apple ][e we had 180 MB 8" drives, but I had the Seagate 5.25" 20 MB full of games. I had fun working with the engineers from Japan from Toshiba, Fujitsu, and Seagate offered me a QA Mgr job, but (ex)wife was afraid to move \$\endgroup\$ Apr 28, 2021 at 10:08
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    \$\begingroup\$ One can't imagine all the complexities of my 8TB drive with variable bit density zones and Reed-Solomon codes , track densities beyond laser printer resolutions, and thin-film vertical recording heads that can last for 10 yrs flying on micrometer air bearings at 30 to 50 KPH with no data errors to the host while managing all the defects on spare sectors transparently until it stops working without warning from the OS. \$\endgroup\$ Apr 30, 2021 at 0:40

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. https://www.cnet.com/news/hitachi-intros-first-7200rpm-single-platter-7mm-hard-drive/

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    \$\begingroup\$ This answer reminds me of the cat with toast taped to its back paradox. \$\endgroup\$
    – DKNguyen
    Apr 28, 2021 at 3:23
  • \$\begingroup\$ @DKNguyen nowhere does it state how big the hard drive is except maybe the series number. If you're using like an oversized flash drive but don't want to pay for an actual oversized flash drive, you might be willing to pay for half the capacity but a tiny bit thinner hardware. And if you're using as a wealth token, doubly so. \$\endgroup\$ Apr 28, 2021 at 16:54
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    \$\begingroup\$ Single-platter and single-sided are two very different things. The head arm assembly clearly has two heads, so the image is of a single-platter, double-sided drive. \$\endgroup\$
    – AnalogKid
    Apr 28, 2021 at 19:43
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    \$\begingroup\$ @AnalogKid Isn't that the point? The OP's quite states that the top and bottom sides of a platter stack are unused. If that were true a single-platter HD could not exist, whether or not it was double-sided or single-sided by virtue of the single platter being both the top and the bottom platter requiring at least one side to be used for data to justify its existence. \$\endgroup\$
    – DKNguyen
    Apr 28, 2021 at 23:11
  • \$\begingroup\$ Are there any hard drives that don't spin? \$\endgroup\$
    – Voltage Spike
    Apr 29, 2021 at 2:49

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