Originally Posted by SimonChambers
SD cards are just a couple I/O lines into the SPI peripheral of the microcontroller.
But that requires the microcontroller to have a free SPI port. As one of the requirements of the 9xr was to be directly compatible with existing 3rd party firmwares for the 9x, the microcontroller and connections had to be the same. Which means no free SPI port, just like on the 9x. Its pins are used for trim keys if I remember well.
Again it's a $50 radio. If you want SD card and actually make use of it with voice prompts and telemetry logging, go see the frsky radios