You might need to make sure the partition type is set correctly, as well as the filesystem format. Though I'm not familiar with that code, so Rockbox might just ignore the partition type.
The differences between SDHC and SDXC:
SDHC: uses FAT32, partition type 0x0c. Up to 32GiB (though that's an artificial limit by the standard).
SDXC: uses exFAT, partition type 0x07. 32GiB upwards. Needs to get reformatted to FAT32 for use with Rockbox.
Apart from those there's no difference between those two that I'm aware of.
Hi... I might want to locate a conclusive answer on what sort of memory cards it employments.
Not sure how a conclusive answer should look like. You'll never know what the industry / vendor <whatever> will sell tomorrow. As it stands right now the only relevant difference for Rockbox is the filesystem it comes with.