The SD card interface is fixed at 3.3V I/O. To support high speed modes (using clocks beyond 50Mhz) you need 1.8V I/O voltage like on the eMMC interface. You could switch the SD card interface to 1.8V on a design but that removes backwards compatibility with slower/older cards that can only do 3.3V I/O. You could implement a dual level I/O scheme supporting both 1.8V and 3.3V but I haven't seen anyone going through with that on a cheap SBC design. If you want high(er) speed storage on Rock64 you need to go with the USB3 port.
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