There are countless examples of devices that use standard components and interfaces, but with custom firmware they can still be locked. Hard drives in media players were the most common one.
DJI can totally justify it by saying that for this price you get reliability, because who would want to have a $8k machine in the air on a set and lose a shot because the cheapo hacked off the shelf SSD can't sustain the rate - the lost revenue on the first occasion it happens will exceed the price of 2 or 3 of their SSDs. Even if they're about 3x too expensive IMO nobody in the business who really has a reason to have an X5R has a reason to take the risk either.
For the record the list price for a 2.4Gbps-capable 512MB Sony AXS-A card (which incidentally has a very similar form factor, can't find info about interface though) is $2120, and the reader is $660.
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