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.
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.)