How can I protect access to an SD card, so that only authorized users can read or modify the content of the SD card. Currently everybody who has physical access to the SD card can read or modify everything.
Simplest solution for protecting against unauthorised viewing or modification: encryption. You can use TrueCrypt, PGP, GPG, OpenPGP etc to either encrypt individual files on the card or to set up an encrypted volume for the whole card.
This will mean only those people who have the passphrase will be able to access the data (given the usual assumptions around correct implementation, no sharing of keys etc)
The latter solution also allows you to effectively securely delete the card, by getting rid of the encryption passphrase (or password or certificate etc)