The Wiki says that the Pro will support SDHC and SDXC uSD cards, but I can't find anything in the Wiki that would suggest the Pro will support the UHS-I (Ultra High Speed) uSD cards, so there's no point in spending the extra money ... is that correct?
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Correct.
The SDcard bus is relatively slow and no faster than the original PinePhone.
Far more important to get a card with fast small file reads and writes. There are threads in the original PP section and RockPro64 section on a similar theme, Samsung Evo+ work for me.
- PinePhone BraveHeart now v1.2b 3/32Gb daily driver, Tow-boot with pmOS/SXMO on eMMC
- PinePhone Pro Explorer Edition, Tow-boot on SPI, Arch/SXMO on eMMC
- ROCKPro64 v2.1 2GB, 16Gb eMMC retired in favour of a fruity upgrade
I trued using an SDHC card but I couldn't get it to boot from the SD card (Waited 45 minutes). You want a faster card (SDXC at minimum).