# Question about hard drive structure... **URL:** \ **Category:** Factual Questions\ **Created:** [November 29, 2009, 11:38am UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325 "2009-11-29T11:38:56Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Saffer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saffer/32/6440_2.png) [@Saffer](https://boards.straightdope.com/u/Saffer)\ **Post date:** [November 29, 2009, 11:38am UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325/1 "2009-11-29T11:38:56Z")
I was just reading something about how hard drives store information. Apparently, data is stored by magnetizing a series of concentric circles called tracks. Each track is track is divided into a number of equal sized wedge-shaped regions called sectors. Each sector, throughout the disc holds the same amount of data, usually 512 bytes. So you can imagine a disc, with a bunch of rings, and a bunch of lines radiating from the center to divide the rings into sectors. The thing that gets me is, in order to have each sector hold the same amount of data, the bytes have to be more closely packed together in the center than at the edge!?:dubious: Would it not be better to maximize the storage capacity by packing them just as close throughout the disc and increasing the sector size as you get closer to the edge? Or is it so much easier to process the data if each sector is the same size that storage efficiency takes a back seat? ---
**Author:** ![mecaenas](https://avatars.discourse-cdn.com/v4/letter/m/91b2a8/32.png) [@mecaenas](https://boards.straightdope.com/u/mecaenas)\ **Post date:** [November 29, 2009, 1:05pm UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325/2 "2009-11-29T13:05:43Z")
Storing gobs of data is fine, but it’s also important to be able to access the data at a consistent speed. > [@](#): > > The stack of platters rotate at a constant speed. The drive head, while positioned close to the center of the disk reads from a surface that is passing by more slowly than the surface at the outer edges of the disk. To compensate for this physical difference, tracks near the outside of the disk are less-densely populated with data than the tracks near the center of the disk. The result of the different data density is that the same amount of data can be read over the same period of time, from any drive head position. [http://www.ntfs.com/hard-disk-basics.htm](http://www.ntfs.com/hard-disk-basics.htm) ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 29, 2009, 1:52pm UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325/3 "2009-11-29T13:52:02Z")
This used to be how disk drives were built. But not for quite some time. As noted above such a design makes the control electronics a lot easier to build - since all the timing is the same. Modern drives will record a constant data density and vary the timing to compensate. However this makes the control systems much messier, and disks much harder to describe. You still have tracks, but a varying number of sectors per track. There is a fiction maintained about sectors per track. Older file systems and disk controllers had the old idea of disk layout built into them. So you may find yourself having to fabricate a disk geometry specification. This usually ends up with the real number of tracks, and a number of sectors per track that results in a capacity that is correct for the disk. But it remains a fiction. ---
**Author:** ![Saffer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saffer/32/6440_2.png) [@Saffer](https://boards.straightdope.com/u/Saffer)\ **Post date:** [November 29, 2009, 1:52pm UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325/4 "2009-11-29T13:52:56Z")
Ok, I never considered the consistent speed issue. Makes more sense to me now. Thanks 🙂 ---
**Author:** ![Saffer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saffer/32/6440_2.png) [@Saffer](https://boards.straightdope.com/u/Saffer)\ **Post date:** [November 29, 2009, 2:01pm UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325/5 "2009-11-29T14:01:51Z")
Francis, are you saying that modern drives do in fact store data in the same density over the whole disk and just slow down to read the outer tracks? I think I understand what you mean about the “fiction”. Are you saying that the software expects the old uniformly sized sectors and we come up with specifications describing the disk as it would be if it were the old design so that the software can control the hardware properly? If I have misunderstood, please clarify. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 29, 2009, 2:14pm UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325/6 "2009-11-29T14:14:22Z")
They can’t slow down the disk - but the drive electronics are now sufficiently flexible that they can cope with sychronising to data coming off the disk surface across a reasonably wide (say 2:1) range of data rates. And can also sychronise approriately to write data. You have the idea of the fictional description spot on. However it isn’t so much that the software can control the hardware properly, just that the software feels happy in its little cocoon. Other parts of the operating system will simply turn the specification back in to a linear space of blocks. It used to be that you asked the disk for platter, track, sector. Now you just ask for a block, where block numbers start at zero and run to the end of the disk. Disk manufactures are remarkably reticent to tell you much about what really goes on inside a disk drive nowadays. ---
**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\ **Post date:** [November 29, 2009, 2:15pm UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325/7 "2009-11-29T14:15:19Z")
> [@Saffer](#): > > Francis, are you saying that modern drives do in fact store data in the same density over the whole disk and just slow down to read the outer tracks? They don’t slow down, they just know the difference between the inner and outer tracks and can read the sectors even though there are more or less sectors per revolution near the edges. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [November 29, 2009, 2:27pm UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325/8 "2009-11-29T14:27:03Z")
> [@drachillix](#): > > They don’t slow down, they just know the difference between the inner and outer tracks and can read the sectors even though there are more or less sectors per revolution near the edges. I believe that would be “more”. I haven’t disected a drive recently, but when I did, the number of sectors was not constant from center to edge. However, the interfacing software was set up to translate from actual to virtual locations and vice-versa so older drivers and opsys could handle it without a serious modification. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [November 29, 2009, 3:01pm UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325/9 "2009-11-29T15:01:10Z")
> [@Musicat](#): > > I believe that would be “more”. > > I haven’t disected a drive recently, but when I did, the number of sectors was not constant from center to edge. However, the interfacing software was set up to translate from actual to virtual locations and vice-versa so older drivers and opsys could handle it without a serious modification. And, how could you tell this by dissecting a drive? Can you see magnetic domains? ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [November 29, 2009, 5:19pm UTC](https://boards.straightdope.com/t/question-about-hard-drive-structure/519325/10 "2009-11-29T17:19:16Z")
> [@beowulff](#): > > And, how could you tell this by dissecting a drive? Can you see magnetic domains? No. I mean disecting the software. If you have access to a translation table, you can see how the physical is mapped to the virtual.