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Just recently a 200 (yes, 200) GB card has been announced by some flash memory manufacturer. The question is not about the giant capacity this card has or how they are made, but the peculiar number on it.

I always thought memory cards had only capacities measured by powers of 2: 32, 64, 128 because of the binary representation of electricity and the subsequent multiplexing of memory addresses… Taking this into account the next card should have been 256 GB by once again doubling the memory cell density, but it’s 200 GB… Any insight on why is this?

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ibelcomputing
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  • Capacities don't have to be limited to powers of 2. I mean 80GB and 500GB were common capacities a few years ago for HDDs. – Ramhound Mar 02 '15 at 16:28
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    Mechanical HDDs, yes. For a MicroSD card I would definitely expected 256 GiB. – ibelcomputing Mar 02 '15 at 16:30
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    You created an artificial restriction in your head. 6x32GB = 192GB + 1x8GB = 200GB. – Ramhound Mar 02 '15 at 16:33
  • Also if you think about it, most 32, 64, and 128 GB cards aren't actually 32, 64, or 128 GB. They are almost always slightly less, because manufacturers can get away with assuming that 1GB = 1000 MB instead of 1024MB. Drive space is usually close to a power of two by custom, not by some technical restriction. – tlng05 Mar 02 '15 at 17:19
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    That's right. Yet while a 32GiB card with a single partition has approx. 29 Gib of useable disk space depending on the filesystem used, they don't advertise them as being a 29 GiB card... This one has 200GiB written on the label, so I now wonder if that will be the case here, adding to the oddity. – ibelcomputing Mar 02 '15 at 17:24
  • This question is opinion based and the answers and comments are basically just theories. – Giacomo1968 Mar 02 '15 at 17:24
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    @JakeGould I want an honest explanation on why is this... It's not just that memory card, I have found some solid state drives have capacities that don't follow this rule as well. There are 120 and 240 GB solid state drives. I also find it curious how there hasn't been a single concrete answer yet a lot of criticism. – ibelcomputing Mar 02 '15 at 17:30
  • **“I also find it curious how there hasn't been a single concrete answer yet a lot of criticism.”** Why do you find that curious? This is a bad, opinion-based question. None of us here manufacture SSD dies in our living rooms. Contact a manufacturer and ask for details. Here you want an answer, check this out: http://superuser.com/questions/747244/why-do-ssds-have-weird-sizes – Giacomo1968 Mar 02 '15 at 17:37
  • @ibelcomputing - I already provided an explanation. I am not going to post an answer indicating your making artificial restrictions in your head ( it wouldn't be productive ). This new device will work exactly like all the other devices. if you take the overhead on the 192GB storage and the overhead on the 8GB you would get the useable space on an advertised 200GB device. – Ramhound Mar 02 '15 at 18:49
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    Then this http://superuser.com/questions/358031/why-does-everything-in-computing-work-with-a-base-number-of-2-to-the-power-of-x question is missing that information. I don't understand the downvotes, I never mentioned brands (I was careful with that) I did research before asking this question. Not knowing about JBOD and manufacturers not using this technique before on microSD cards don't make this a bad question, and it's certainly NOT opinion based. Now it's closed as off topic, it's not about computer hardware? Seriously??? Can someone please raise this to meta? – ibelcomputing Mar 02 '15 at 19:53

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It might not apply to your SD card, but I have seen very cheap USB sticks with unusual capacity and the reason was: the production could not ensure the quality of all 128 MB (yes, that was MB time), so the capacity was set to whatever could be ensured during the testing process.

Applied to your 256 GB SD card, that would mean: it is produced with 256 GB in mind, then the defective parts are crossed out, leaving you with tested and guaranteed 200 GB (+ perhaps some spare GB for self recovery).

Thomas Weller
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Use existing technology of 128GB, plus an optimized way of holding 64GB and 8GB. 128 + 64 + 8 = 200

Kirill2485
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  • Do you know for a fact that this is how they're made, or is this a guess? – fixer1234 Dec 09 '15 at 23:54
  • it's kinda obvious, but if you wanna see for urself, here u go: https://www.sandisk.com/about/media-center/press-releases/2015/200gb-sandisk-ultra-microsdxc-uhs-i-card-premium-edition, notice how it sais "per die" in "128GB SanDisk Ultra microSDXC card, and creating a new design and production process that allows for more bits per die. " and the only way to create the layers in the dies from 128gb to make it 200gb is by optimizing the way i optimized it – Kirill2485 Dec 10 '15 at 01:28
  • That's not the only way; not sure it's a likely way. – fixer1234 Dec 10 '15 at 02:18
  • 128gb + what + what = 200 then? nothing except 128+64+8. this prooves that it is the ONLY way, and you cant do 128+72 because 72 doesn't exist. Think you have a great solution for how to add numbers together to equal 200gb? Go for it... i wanna see u try – Kirill2485 Dec 10 '15 at 02:34
  • unless you meant another way they can do it would be to do 128+16+8+8+8+8+8+8+8, but then that would be stupid, that would just take up more space. – Kirill2485 Dec 10 '15 at 02:40
  • 1) They can make the die whatever they want, and not necessarily by adding incremental chunks. 2) It has a very long warranty. They could have doubled the size and used the excess for spare blocks and initial failed blocks. – fixer1234 Dec 10 '15 at 02:41
  • Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/32769/discussion-between-kirill2485-and-fixer1234). – Kirill2485 Dec 10 '15 at 02:41