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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Gordon
The filesystem currently only runs SPI at 1M baud, so that's unlikely to be the problem. You can actually already set the baud rate manually just by using the filesystem once, then just running SPI2.setup again with the new baud rate.
I still think not using the voltage regulator is a bad idea. A fully-charged NiMh battery (which could be likely if charging from solar power) is around 1.4v, so 3x = 4.2v which may well destroy your SD card (and which is also too high for the STM32).