My general impression is that SDHC support implies SDXC support, even if it doesn't say so on the tin.
Yup, unlike the plain old SD card format (which was limited to 1GB due to a small number of addressable blocks. Up to 4GB by using larger block), the protocol hasn't changed at all between SDHC and SDXC. The difference is purely software:
SDHC are formatted with FAT32, whereas the SDXC standard mandates the use of exFAT. Which Microsoft has patented the shit out of.
Any slot can access both SDHC and SDXC cards without any distinction.
The limitation is at the *OS level*, and depends on whether the OS maker has paid the necessary patent tax to be able to access the logical content of the card: An SDXC slot is simply an SDHC slot on a device whose OS has a driver for exFAT in addition to FAT32, etc.
You can use a SDXC card in any device advertised as SDHC-only only simply by :
- installing an exFAT driver (e.g.: install FUSE-exFAT on Sailfish OS)
- or reformatting the card with something that the OS supports out of the box. Some Android devices and photocamera will automatically give you the possibility to reformat the card. Other device (like Nintendo's New 3DS) will require you to manually reformat the card using a separate device before plugging in.
The size will have absolutely NO influence. (Again, that's unlike plain SD card, which use an older protocol that can only reference a smaller number of blocsk)
They often come up well short of the rated speed of the SD media, but they still work.
And that has nothing to do with SDHC/SDXC format or the size.
That's basically similar to all the various UDMA mode available on older IDE (parallel ATA), 16bit PC-Card and Compact Flash cards.
There are several different speed protocols available for SD cards.
On your device, the SDXC card fall back to older and slower speeds (Class-10, class-6, etc.), whereas the SDXC could have supported a faster one (UHS-1, UHS-3) had the reader had it too.
At least that's the theory, when writing on a plain empty card.
In practice, as there are already data on the card, it is limited mostly by the read-erase-write cycles and various wear-levelling tricks.
(So it's mostly due to an interaction between the file system used by the OS and the firmware running on the SD card.
- With Log-Structured and Copy-on-Write filesystems like UDF, F2FS, BTRFS, ZFS being better than classical FAT32.
- And SD cards capable to handle many allocation units in RAM at the same time performing better)