SD Spec contains some secure features that are not easily reproducible when just imaging through standard user mode apps. Might pay to have a read of the spec.
Commonly used in things like GPS Data add on packs for Sea Vessels, and other software packs for certain devices where you have to pay mucho $$$ and copying them is not a scenario the vendor wants
Also recently there was a discussion about cash register cards from (IIRC) Italy where the data had to be saved to an authorised SD Card for the purpose of taxation.
Is your card in the category of software or special purpose? or just a user card that is suspicious?