I want an internet connected host, such as a Raspberry Pi, to be able to write uploaded files to a directly-connected SD card, but I don't want it to be able to read the card. This is for protection in the event that the host is compromised.
In the Simplified Physical Layer specs from the SD Association, it refers to "dual simplex" as a feature of the SD Express Card (connected via PCIe). See page 24 (document page 4) here https://www.sdcard.org/downloads/pls/click.php?p=Part1_Physical_Layer_Simplified_Specification_Ver8.00.jpg&f=Part1_Physical_Layer_Simplified_Specification_Ver8.00.pdf&e=EN_SS1_8
Unfortunately there are no other references to simplex in the spec. Is is possible that, by disconnecting one of the simplex channels, I will be able to achieve my goal? I realise that there may be some trade-offs, particularly where file system commands are concerned. For example, the host may not be able to do a directory listing of the card.
I realise the SD card can be read-protected with a password, but that's not acceptable for my use case. (The possibility of a back-door always exists).