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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.
Vicary
Hi @Vicary, this is a lot more difficult than it sounds. I investigated emulating a SD card (not with Espruino) and it was a lot of work. I found a solution for my needs (security testing) by getting a SD->USB Mass Storage adapter, and then emulating USB mass storage with a microcontroller. So it went Microcontroller(emulating USB MS)->adapter->MicroSD->Device
It was pretty slow, and the adapters were hard to find and now discontinued:
http://www.saelig.com/product/mio001.htm
Only other option I saw at the time was an expensive industrial SD card testing unit. I might have another look because it would be so useful.
An easier solution for you might be to use a USB drive to hold your root file system. Emulating a mass storage device is much easier, and I believe there might be devices that allow you to do it (I have one which selects ISOs and emulates a DVD drive).
See https://samhobbs.co.uk/2013/10/speed-up-your-pi-by-booting-to-a-usb-flash-drive