Hard disk surface

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

Ask QuestionAsked todayActive todayViewed 189 times2

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?

Here you can see it. 

Also you can see the same thing in this link.computer-architecturehard-driveShareCiteEditFollowFlagasked 11 hours agoANSHUL GUPTA4933 bronze badges

  • 3I’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. – jonk 11 hours ago 
  • 2I 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. – Elliot Alderson 10 hours ago
  • 5@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? – SamGibson 10 hours ago
  • @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.) – jonk 10 hours ago
  • I’ve never seen a hard-disk platter without a head on each side. The balanced pressure is critical. Who knows if the top platter’s outside-head is actually used for data? – glen_geek 10 hours ago
  • 2@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 😦 – SamGibson 9 hours ago 
  • @glen_geek Now you just reminded me of an old system with multiple platters and only one pair of heads that had to be moved from platter to platter…. and others with one head per track… and… that I didn’t even see any with low mass/low load heads and lubrication until the 1970’s. Oh, well. Now I’m remembering the drum drives (and one time when I just kicked one — a common remedy — to get it working again.) – jonk 9 hours ago
  • @SamGibson Well, so long as there is some successful way to argue that the comment suggests a constructive approach to improving the question, even if that isn’t all of it, then perhaps it can stand as such. I know contributors here feel just fine dashing off short-shrift answers. I don’t because my view is always a long one — years and years out — and not only about some quick answer only useful to the OP and to no one else, later. It’s not that I won’t write something direct. But if I do I also want to make it still useful for deducing into different circumstances, too. More work. – jonk 9 hours ago
  • Those references are outdated and unsupported by companies and poor descriptions of HDD’s – Tony Stewart EE75 7 hours ago
  • @jonk, me ok-boomer also saw almost everything, though forgot almost all. I remember I read from text books when doing a rusty EE diploma, magnetic tape, which stores more stuff than disks, but sequentially and so takes long time to search for something. I guess by now all tapes have been replaced by HDDs, and not too soon HDDs would be replaced by SSDs. (I have 3 cheapy hobbyist grade Samsung T5 1TB SSDs now. So I tried to give way my old 300Mb HDDs, not too long ago, to my 12 yr niece, but she stared at me with a strange look at her face, said she wants 128G fingers to store her TikToks: (. – tlfong01 3 hours ago   
  • @tlfong01 Oh, heck. I spent years mounting magtapes, doing full backs, incrementals, and restores. Also used DECTape, papertape, vibrating mercury memory in long tubes, etc. A washing machine sized box was 8k of drum memory, for example, in 1972 or so. – jonk 2 hours ago

Add a comment

2 Answers


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/ShareCiteEditFollowFlaganswered 5 hours agoPasserby64.9k55 gold badges7272 silver badges179179 bronze badges

  • This answer reminds me of the cat with toast taped to its back paradox. – DKNguyen 3 hours ago

Add a comment4

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.ShareCiteEditFollowFlagedited 6 hours agoanswered 7 hours agoTony Stewart EE75109k33 gold badges3939 silver badges148148 bronze badges

  • 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! 😦 – tlfong01 2 hours ago    
  • (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. – tlfong01 2 hours ago    

Add a comment

Categories: Uncategorized

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.