Is it possible to use Arduino (or any other MCU) to intercept SPI communication between Raspberry Pi and external SD card ? I would like to intercept CMD24 and CMD25 of SD card SPI protocol to fail if the block is already written - implementing WORM storage.
Update: I need hardware WORM block device (filesystem agnostic) - essentially WORM SD card (which is expensive and practically impossible to obtain). So I was thinking about cheap MCU with SD card reader that would provide WORM access to regular external SD card through SPI interface:
[RPi or similar with SPI interface] <===SPI===> [MCU] <===SPI===> [SD card reader]