Use BoxCryptor. It has companion applications for Windows, Mac, iPhone & iOS. You can keep your encrypted files in cloud (Dropbox, Google Drive or Microsoft SkyDrive) & access them on any platform.
I use it on regular basis for offline files too (what you are looking for). I keep encrypted container of BoxCryptor in my Dropbox and use FolderSync two-way sync to take all files offline on SD Card. Then, I use BoxCryptor to decrypt those files locally whenever I want.
If you don't want to use cloud, just don't use it. Choose SD card in BoxCryptor app. After inserting the SD card to PC or Mac, one needs BoxCryptor applications of Windows, Mac (Encrypted files are compatible with
EncFS-Linux too) & password to read those files & folders (file names & folder names are also encrypted).