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Like many people, I've always though that sdcard lock is physical protection. But I've just bought cheap sdcard reader which doesn't really care much about it. It made me realize that it's not physical protection and can be easily bypassed by infected OS. As it contains really important data - is it possible then to make sdcard REALLY read only medium? I've chosen memory card because I need storage easy to be destroyed physically and difficult to be destroyed programmatically.

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    Based on the answer here: superuser.com/questions/354473/… - not possible due to how SD card is implemented. Even their specification said "It is the responsibility of the host to protect the card. The position of the write protect switch is unknown to the internal circuitry of the card." So that switch can be ignored as easily by the host as there is no physical disconnect. You may be better off using CD-R (or if you prefer a smaller medium, Mini CD). – Darius Aug 12 '15 at 23:34
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    I'm going to make some guesses. If the computer is responsible for honoring read-only, a malicious computer could ignore. I believe an SD card performs some actions itself, so a custom card could, in theory, ignore bad commands from the computer. However, there would be a cost to that, so common/economical/cheap SD cards wouldn't do this. IIRC, I have a friend who broke a tab, causing SD card to be read-only. By modifying his OS, he managed to write to the card anyway. So violating the design specifications ended up being useful in practice. – TOOGAM Aug 13 '15 at 0:46
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SD Specifications Part 1 Physical Layer Simplified Specification, section 4.3.6 "Write Protect Management"

[...] It is the responsibility of the host to protect the card. The position of the write protect switch is unknown to the internal circuitry of the card.

So, an infected system could potentially ignore the write protection. I'd use something that can't be physically rewritten like CD-R or DVD-R if you need more storage. Dual-layer DVDs can have around 8 GB of usable space. Discs can be easily destroyed (just try to fold it in half, it'll break. You might want to wear safety glasses or something.)

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    I'm afraid CDs are easy to be recovered with professional tools eg. by government or police. Micro sd can be literally turned into dust in few seconds or burnt with fire or excessive voltage - it's just tiny piece of plastic. I guess I'll just build own usb read only card controller then if it's necessity. I though it'll be easier like dunno isolating one pin, but as far as I looked at pinout it's not possible to be done easily. – Lapsio Aug 13 '15 at 1:38
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    You could ruin a disc beyond government repair but with excessive heat/voltage too, but I suppose it would be easier to do that to a memory card. – danielcg Aug 13 '15 at 1:40
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    If you're concerned the government and police are coming for your data, you're probably better off making a rig to destroy CDs securely than reinventing the SD card... – Michael Bailey Aug 13 '15 at 5:21
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    You have a rig for destroying CDs in the kitchen - it's called a Microwave ;) – Tetsujin Aug 13 '15 at 16:10

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