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I am just asking, whether it's technically possible for a SD card to be SD XC, but have SD-HC or SD-NC (normal capacity) capacities, which are well below 64GB.

Of course; in reality, there is no reason for a higher SD card standard to have less storage.

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    Of course there could be a reason: speed and reliability. Nobody would pay for it probably though. – PlasmaHH Feb 19 '18 at 10:29
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    @AlexanderKosubek: Do cards that big exist? I'm not even sure I've seen SSDs that big... – Mehrdad Feb 19 '18 at 22:54
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    @Mehrdad The largest SD card I was able to find was a 512 GB card, so we're getting there. 2 TB, and even 4 TB SSDs definitely exist. – jrh Feb 20 '18 at 3:12
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    @jrh: The weird thing is there's been news of this beast since 2009. – Mehrdad Feb 20 '18 at 3:57
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    My first comment, asking if theTechLord had "2 GB" confused with "2 TB" was based on a misunderstanding: I didn't initially realize, that the question was about if a newer standard card could have a lower capacity and still be compliant to the newer specs. - So, my apologies! - I posted a new comment to clarify on @Mehrdad's f-up, though. – Alexander Kosubek Feb 20 '18 at 10:03
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Yes, technically

There are three separate things conflated here:

  • The protocol (the SD Physical Layer specification that applies - now up to v6.0) implemented by the SD to NAND flash controller chip
  • The amount of NAND flash memory attached to the controller chip.
  • The factory formatting of the card (FAT16/FAT32/exFAT)

The SD/SDHC/SDXC label is marketing that simplifies the above. The main differences are:

  • SD: the original. FAT16 formatted. Can be up to 4GiB but in practice usually 2GB is the upper limit.
  • SDHC: 4-32GiB. Some protocol changes (sec 4.2.3 and 4.3.14). Typically needs firmware changes (eg in a USB card reader) to support. FAT32 formatted. Most 4GB cards are SDHC, even though the spec says SD can be up to 4GiB.
  • SDXC: exFAT formatted. No protocol changes I could see in the spec. Cards above 32GiB are SDXC.

In other words the hardware/firmware is the same for SDHC and SDXC, it's just the pre-written data that's different.

Now, there's no reason you couldn't fit a 2GiB flash chip to a SDHC controller, format it as exFAT and call it SDXC. You lose the ubiquity of FAT (users of exFAT have to pay a patent fee to Microsoft, hence it isn't so common) but the extra features of exFAT may make up for it. The SD Association might complain about confusing marketing if you tried to sell it, though.

That said, the formatting of the card is just a case of the data written to it in the factory. If you reformat a 2GB SD card to exFAT you achieve almost the same effect. Arguably it's better since it will work in some old card readers and such whose firmware doesn't support the SDHC protocol.

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    SDHC and SDXC being the same at the protocol level is a fascinating claim that I see made explicit here for the first time. It is however consistent with my experience (use of cards larger than 32GB in devices that use non-Microsoft filesystems yet do not claim SDXC compliance, such as Raspberry Pi computers), so I should have guessed. – dhag Feb 19 '18 at 15:09
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    Interesting; my Sony camera demands SDXC for full-bitrate video, yet I don't want to waste 64 GB on it. I have fast (U3) 16 GB cards, yet it rejects them. I shall try to format them in exFAT and see if it fools the camera. But something tells me that, as usual, Sony is more... sneaky. – Zeus Feb 20 '18 at 5:52
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The SD association states that an SDXC card has more than 32GB capacity on their website: SDcard.org

This seems to indicate that a smaller card isn't an SDXC card. Of course, the change from SD to SDHC and then to SDXC wasn't only about increasing the capacity.

For example, each new version also introduced faster transfer modes and even within SDHC and SDXC there are multiple upgrades to the standard. The interesting thing is that often these upgrades were also applied to the previous capacity standard. For example, adding the UHS (ultra high speed) bus was done for both SDXC and SDHC cards: Wikipedia on SDXC

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    SD-HC also can have 32 GB but at maximum. – neverMind9 Feb 19 '18 at 9:30

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