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Put the SD card in Read only mode. Save yourself $10.

Not enough by a far margin.

There's a computer inside that SSD that reads a soft signal to decide if writing is enabled or not. More often than not its firmware is full of known bugs.

Correct me if I'm wrong, but that's because SD cards are half duplex, right? So there's no way to disable writing through a "disconnect pins x and y" because those pins are needed for reading.

SD cards implement a simple four-pin mode (SPI) for which there's public documentation, and a proprietary mode parallelizing data over more pins. I know more about the former than the latter, but it's probably a similar situation.

SPI is full duplex; it has a dedicated line per data direction (MISO and MOSI), however the same lines are used for commands and data. So with MOSI (master out, slave in) physically disconnected, it'd be impossible to send the command asking to read data, even though that data would be delivered on the separate MISO (master in, slave out) line.

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