How can I encrypt an SD Card that already has files on it on my 9320?

I encrypt an SD card which has existing photos using 9320. When I test the card in 9270 the photos show up! When I take a fresh photo using 9320 it gets encrypted and will not show on 9270. My question is are only new files added after encryption is enabled get encrypted or can one encrypt an SD card which already has files on it?

I have formatted the SD card and then added existing photos from PC to 9320, yet when I test on 9270 the files still show up even though I have encryption turned on in 9320.

I am new to encryption so I must be missing something really obvious so any help would be appreciated.