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Author Topic: Are "1TB" micro-SD cards 1000GB or 1024GB?  (Read 594 times)

Offline Frankenpod

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Are "1TB" micro-SD cards 1000GB or 1024GB?
« on: May 20, 2025, 09:06:56 AM »
I've been experimenting with how close one can get to the 2TiB limit (using the iFlash quads in ipods) and am confused by the results I've gotten.
  Based on previous results I _had_ concluded that so-called "1TB" cards are actually 1000GB - which would seem to be consistent with how storage media are labelled using the "SI" prefixes, based on factors of 1000, rather than the 1024 multiples computing usually uses (hence the distinction between GB/TB and GiB/TiB etc).

  But when I tried a combo of two 1TB cards, a 128GB and a 64GB, it came out, when restored, as 36GB.  To me that seems consistent with the 1TB cards actually being 1024GB, which would mean the total adds up to 2240GB, which is about 2086GiB, so about 38GiB over the maximum 2048GiB.  Had the 1TB cards been 1000GB the total would have been 2192GB, i.e. 2042GiB, which should have worked as it's under 2048GiB.

  (With the 64GB excluded card it worked, but in the end rearranged things entirely to make two separate ipods each with 1TB 512 512 128 combo, which always seems to work)

So are 1TB cards actually 1024GB?  That seems a bit weird to me, given that in general the card manufacturers seem to use the decimal definitions of the prefixes.  Is it possible some manufacturers' 1TB cards are 1000GB but others give you the 'full' 1024GB?
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Offline gevaerts

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Re: Are "1TB" micro-SD cards 1000GB or 1024GB?
« Reply #1 on: May 20, 2025, 09:58:29 AM »
My guess is that "it depends on the manufacturer" is the correct answer.

That said, trying to get exact card sizes by trying out combinations of them and seeing what they add up to feels like the hardest way to do it. I'd just put them in a card reader and see what they actually report as.
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Offline Frankenpod

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Re: Are "1TB" micro-SD cards 1000GB or 1024GB?
« Reply #2 on: May 20, 2025, 01:30:57 PM »
Yeah, you're right, that just looking at them in a card-reader would (have been) the way to go.

  Just didn't really think about the question beforehand, because I'd concluded (judging from how these ipods turned out before with various card-combos) that 1TB must be 1000GB, but then found that (1000,1000,128,64) combo didn't work, and can't really check the issue after-the-fact as the cards are now all in assembled-and-synched ipods (that I'm not dismantling again!).

Think I'll take as a working assumption that different manufacturer/brands define 1TB differently.  Haven't seen any results from anyone checking them, though.

[edit] There seems to be no consistent relationship between the 'nominal' capacity of the four-card combo based on the claimed card sizes, and the capacity you eventually get.  E.g. for ipods I have using (1TB 512 512 128), (with a hotch-potch of different card brands) the eventual size reported by rockbox for them ranges between 1989GiB and 2024GiB
« Last Edit: May 20, 2025, 02:00:54 PM by Frankenpod »
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Offline speachy

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Re: Are "1TB" micro-SD cards 1000GB or 1024GB?
« Reply #3 on: May 20, 2025, 03:58:51 PM »
Quote from: Frankenpod on May 20, 2025, 01:30:57 PM
Think I'll take as a working assumption that different manufacturer/brands define 1TB differently.  Haven't seen any results from anyone checking them, though.

I would be *shocked* if any SD card was anything other SI prefixed.  The storage business has been nearly entirely SI-prefixed for what, 40+ years now?

(The only general exceptions I'm aware of are old floppy drives and CDs, those used MiB.  Hard drives and DVDs have always been SI units)

FWIW, I have a card labeled as "16MB" card that is actually only 14.9MB (or 14.2 MiB), similarly an "8GB" card reports as 7.95GB/7.40GiB.

So clearly that capacity on the label is only nominal, precise capacity can and will vary.  But I can promise you it's going to be SI units either way.
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Offline Frankenpod

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Re: Are "1TB" micro-SD cards 1000GB or 1024GB?
« Reply #4 on: May 20, 2025, 05:52:34 PM »
Thanks for that information.  So the issue is more likely to be just cards not being quite the capacity they claim to be, even if they are in theory using SI units?  I don't really know anything about the topic of memory-card-manufacturing, is there a degree of random variation in exact sizing? (leaving aside the issue of outright fakes).  Seems as if somehow the cards in this instance were a tiny bit _larger_ than claimed.
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Offline speachy

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Re: Are "1TB" micro-SD cards 1000GB or 1024GB?
« Reply #5 on: May 20, 2025, 06:17:49 PM »
Quote from: Frankenpod on May 20, 2025, 05:52:34 PM
Thanks for that information.  So the issue is more likely to be just cards not being quite the capacity they claim to be, even if they are in theory using SI units?  I don't really know anything about the topic of memory-card-manufacturing, is there a degree of random variation in exact sizing? (leaving aside the issue of outright fakes).  Seems as if somehow the cards in this instance were a tiny bit _larger_ than claimed.

While the physical flash chips _probably_ have a power-of-2 array of individual cells, that's the _raw_ size.  Some of that space ends up getting used for other purposes than storing user data -- eg ever-increasing amounts of error correction, the flash translation layer (maps a logical sector to a physical one, eg for wear leveling and improved write speeds), and even the runtime firmware for the flash controller itself.   This overhead is fixed for a given design.

Then consider manufacturing testing. If you find a defective block, instead of scrapping the part you could just permanently mark it as bad in the FTL so it never gets used.  How much gets marked off depends on the individual part, but as long as you're still above your target capacity, great!  Otherwise you could just mark off a bunch of "good" sectors and sell it as the next-sized lower-sized part (eg 500GB instead of 1TB). This practice is called "binning"

What's left after all of that overhead and binning is the capacity that gets reported to the outside.
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Offline Frankenpod

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Re: Are "1TB" micro-SD cards 1000GB or 1024GB?
« Reply #6 on: May 22, 2025, 07:39:13 AM »
I admit this is increasingly not a Rockbox-specific topic.  But how 'smart' are SD cards?  Will they gradually reduce in capacity as sectors get marked 'bad'?  I was under the impression that SD cards are significantly 'dumber' than CF cards with regard to that sort-of-thing.  Just am puzzled why I end up with such different total capacity with combinations of what are nominally the same capacity cards (which makes it tricky to get as close as possible to the 2TiB limit without going over it).
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Offline gevaerts

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Re: Are "1TB" micro-SD cards 1000GB or 1024GB?
« Reply #7 on: May 22, 2025, 08:50:05 AM »
I suspect SD cards *used* to be dumber than CF cards, but I also suspect that that was a matter of having the space (and heat capacity) for a fancy controller. These days everything is smaller and less power hungry, so I don't think there's a real reason to not have similar fancy logic in an SD card, at least a higher end more expensive one.

I'm speculating though
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Offline saratoga

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Re: Are "1TB" micro-SD cards 1000GB or 1024GB?
« Reply #8 on: May 22, 2025, 10:37:19 AM »
Quote from: Frankenpod on May 22, 2025, 07:39:13 AM
I admit this is increasingly not a Rockbox-specific topic.  But how 'smart' are SD cards?  Will they gradually reduce in capacity as sectors get marked 'bad'?  I was under the impression that SD cards are significantly 'dumber' than CF cards with regard to that sort-of-thing.  Just am puzzled why I end up with such different total capacity with combinations of what are nominally the same capacity cards (which makes it tricky to get as close as possible to the 2TiB limit without going over it).

They do at least some wear leveling so you shouldn't have loss of capacity until essentially the whole array is finished and the controller can't find any remaining good block to map to a new write request.  I think at that point they stop working or go read only.
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Offline Frankenpod

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Re: Are "1TB" micro-SD cards 1000GB or 1024GB?
« Reply #9 on: June 07, 2025, 09:20:14 AM »
Having had a reason to re-open some ipods, I checked the individual cards from it in a card reader, and it it seems that Sandisk "512GB" cards are reported as 477GiB (which is what I'd expect from a simple math conversion, i.e. a factor of (1000/1024)^3 ) while Integral "512GB" cards come out as 461GiB.  Got the same figures for a few cards of each type.  Not really making any point or asking about it, just putting the information out there if anyone finds it useful to know.  Am now curious to find what other brands/capacities come out as.

Edit - needed to open another one, and again, the Integral was 461GiB and the Sandisk Ultra is 477.  Seems clear Integral are not quite the capacity of Sandisk.  The small differences aren't a big deal (I wouldn't shun Integral cards for the sake of that 16GiB).  The only thing is, it seems, the variation can make the difference as to whether you go over 2TiB (causing complete failure!) or not.

Have Samsung, Patriot, and Amazon Basics cards in other ipods, but haven't had any reason to open them up.  The one with Patriot cards in it seems suspiciously on the low side, so maybe those are like the Integral ones, a little smaller than the norm?
« Last Edit: June 08, 2025, 02:36:41 PM by Frankenpod »
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Offline iPodVT

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Re: Are "1TB" micro-SD cards 1000GB or 1024GB?
« Reply #10 on: June 07, 2025, 07:43:42 PM »
I've got a 512GB Samsung Pro Plus microSD in a thin 160GB 7th Gen iPod.  Rockbox reports 477GiB.
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