Probably using SD's DRM Mechanism (Score 5, Informative)

I've been studying SD cards for the last few months and I've managed to dig up some heretofore "secret" leaked documents about SD Digital Rights Management mechanism and I think I know how such a permanent modification could be performed.

One of the things that all SD cards support is the ability to designate a certain portion (which can include ALL) of the card's block storage as "secure". Once designated as secure, the blocks in question cannot be read, written to, or the area resized without performing an authentication step with the card. This authentication step is known as "AKE".

I'm willing to bet that the phone is using this "secure" facility and marking the entire card, or some significant portion thereof, as a secure storage area.