I’m generally very pleased with the RockPi S and currently (mis)use it as router for my 100M Internet connection at home. It pushes >10MB/s through a Wireguard VPN tunnel without breaking a sweat and runs other crap on it at the same time easily. Great performance for the price. 
Anyway, I just bought a new SD card to replace the cheap 64GB card I had before because under high contention the system tended to get unresponsive due to heavy thrashing/swapping sometimes and I suspected it’s because of the slow SD card (I was right). So I got the Samsung EVO Select 128GB micro SD card, which should be supported according to the hardware page: https://wiki.radxa.com/RockpiS/hardware/rockpiS. It’s an UHS-I card as defined by the SD 3.01 standard, which also should be supported according to the reference manual (page 181): https://dl.radxa.com/rockpis/docs/hw/datasheets/Rockchip%20RK3308TRM%20V1.1%20Part2-20180810.pdf
But when I plug the card into the slot and do a reset nothing happens. Usually at least the blue LED starts blinking, even if the system won’t boot properly, but with the Samsung SD card it’s just dead (green LED is on though). I suspect that voltage switching might not actually be implemented and the card has no fallback mode or something like that? I certainly don’t expect the 100MB/s throughput I benched on my desktop on the SD card interface, but it would be nice if it was readable at all. For now I’m using it with an USB card reader to store the root fs and even that way it’s much, much faster and stable under load than with the old card. It’s the random R/W I care about. I just wish I didn’t have to plug in two different SD cards to even boot it (bootloader, kernel, etc. are still on the old card). My home network setup already looked ridiculous enough as it was 
So is my suspicion right that such cards aren’t supported, or is my card just broken somehow? If such cards aren’t supported it would be nice if it was mentioned somewhere, though. While it’s usable I would have bought a different one had I known it doesn’t work in the built-in slot. But I’m not even sure if it’s supposed to work or not. I would be grateful if someone knowledgeable could at least tell me if it’s the Rock Pi S, or the SD card that’s the issue. Thanks.