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I bought a new 32GB microSD card. Windows says the capacity is 29.3GB (31,474,057,216) in Properties for the drive. Why?

I expected 1024x1024x1024x32 = 34,359,738,368 bytes. I can accept 0.5GB for the file system, but not 2.7GB. There is no logical reason why the addressable space would follow base 10, since the addresses are in binary. (Or are they?)

Chloe
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  • Turn on view hidden files. Is there some kind of "free" encryption software that came with it? I noticed that some of my microSD cards come with hidden software packages. Since you bring it up that reminds me of discussions of "bloat ware" that came up in late computer science classes. – ejbytes Jun 04 '16 at 05:12
  • Please read the comments you already received here and in your other question before posting this same question elsewhere. – Julie Pelletier Jun 04 '16 at 05:19
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    You asked this question once before [here](http://superuser.com/questions/1084265/why-dont-microsd-have-the-full-free-space-they-claim-to-have). Please be sure you do a proper amount of research and verify your question has not been asked before. Having multiple questions closed, and not well recieved, isn't good – Ramhound Jun 04 '16 at 15:16
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    This really isn't a duplicate of the other question. Flash memory, like almost all other memory, [is sold in binary multiples](http://superuser.com/questions/235030/why-are-ram-module-capacities-in-powers-of-two-512-mib-1-2-4-8-gib) regardless of the external interface. – Ignacio Vazquez-Abrams Jun 06 '16 at 20:06
  • @IgnacioVazquez-Abrams I hate to say it but you are actually wrong. The [Sandisk microsd website itself](https://www.sandisk.co.uk/home/memory-cards/microsd-cards/extremepro-microsd-uhs-ii) has a disclaimer at the bottom which states: "**1GB=1,000,000,000 bytes. Actual user storage less.**" They are not sold in binary multiples but follow the same misleading metric units as hard drives. – Mokubai Jun 07 '16 at 18:08
  • Additionally their [SSD page](https://www.sandisk.co.uk/home/ssd/ssd-plus) disclaims "1MB=1,000,000 bytes.", and their [portable SSD](https://www.sandisk.co.uk/home/ssd/extreme-900-ssd) page "1TB=1,000,000,000,000bytes . 1GB=1,000,000,000 bytes." – Mokubai Jun 07 '16 at 18:41
  • @Mokubai: Richard's latest comment made me realize something: We are answering similar yet distinct questions. He answers "why are they marketed with less space than they seem to have", and I answered "why do they claim less space than the underlying media should contain". The question was originally asked where the second interpretation would make sense, yet for some reason was migrated to somewhere the first would apply. – Ignacio Vazquez-Abrams Jun 07 '16 at 18:49
  • @IgnacioVazquez-Abrams I admit I was hasty in closing it as a duplicate of a question specific to hard drives, but I did so because that is where the largest amount of capacity is "lost". It is lost primarily because of manufactured bad blocks, but by using a metric measuring method they can state a capacity they can legitimately expect to be closer to without actually lying. In this it is effectively the same as a hard drive. This question will serve as a good duplicate for the future as it has *both* factors as answers, even if a single answer stating both causes would be better. – Mokubai Jun 08 '16 at 16:46
  • Does this answer your question? [Why are hard drives never as large as advertised?](https://superuser.com/questions/504/why-are-hard-drives-never-as-large-as-advertised) – Chris Rogers Sep 14 '20 at 17:20
  • Also https://superuser.com/q/71974/581166 – Chris Rogers Sep 14 '20 at 17:21

2 Answers2

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If you check the fine print, you may find that 1M is defined as 1,000,000, not 1,048,576. And 1G is defined as 1,000,000,000, not 1,073,741,824

This has been a trick used by hard drive manufacturers for decades. And it appears that the memory card people have picked up the same trick from hanging out with their sleazy hard-drive marketing friends.

Richard Crowley
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  • So you are saying the manufacturer committed fraud? How can a microSD card selectively remove 525,942,784 bytes? That's not even a factor of 2. – Chloe Jun 06 '16 at 06:32
  • It isn't "fraud" if they cover they deception in the fine print. Nothing is "removed". It was never there at all. IMHO, using a different definition for common terms like "mega" and "giga" is deception. YMMV. If it weren't deception, Chloe would have never started this discussion. – Richard Crowley Jun 06 '16 at 11:09
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    Except memory, even flash memory, is always sold in a binary multiple. Marketing isn't the reason. – Ignacio Vazquez-Abrams Jun 06 '16 at 20:03
  • @Ignacio Vazquez-Abrams, you haven't read any fine print lately. I'm holding in my hand four SDHC cards that are 64,000,000,000 bytes. Not a binary value in sight. – Richard Crowley Jun 07 '16 at 05:13
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    That's how much space the SDXC [translation layer](https://flashdba.com/2014/09/17/understanding-flash-the-flash-translation-layer/) is telling you the device has, not how much space the underlying flash memory has. – Ignacio Vazquez-Abrams Jun 07 '16 at 05:53
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    No. That is what is printed on the package from the manufacturer. Go buy one and read it for yourself. – Richard Crowley Jun 07 '16 at 06:12
  • Solid state memory is not like a disk drive. Hard drives have platters, cylinders, and sectors, and could be any number of bytes based on what fits. Solid state memory uses transistors. I find it highly unlikely exactly 64,000,000,000x(however many transistors per bit) transistors fit on your card. That still doesn't explain why my microSD card had 31,474,057,216 instead of 32,000,000,000. – Chloe Jun 08 '16 at 06:12
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The underlying media (flash memory) is measured in binary units, but flash cells become damaged over time. The "leftover" space is used to balance wear so that the media lasts longer than each individual cell.

Ignacio Vazquez-Abrams
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  • But wouldn't the wear be spread over the available capacity by the OS? Are you saying there is a logic controller inside the microSD which allocates writes across the full 32GB, but reserves a hidden 2.7GB from view from the OS? Is this common? The manufacturer is 3C Pro. – Chloe Jun 06 '16 at 06:37
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    The OS does no wear leveling; all it sees is a filesystem. This is universally prevalent for flash devices that show a popular (e.g. UMS, SATA, etc.) interface rather than allowing direct access to the memory itself. – Ignacio Vazquez-Abrams Jun 06 '16 at 06:39