Odd behaviour with compact flash card

[PCUK]

Took some pics and copied them onto the Pad. Put the card back in the camera and got a message saying card not initialised, so couldn't use the card again. Out in a smartmedia card and took some more pics. Same thing happened. PIcs all safely copied onto pad but unable to use card in camera. Back home put each card in the Mac card reader and was able to copy the pics onto the Mac in the normal manner. Out the cards back in the camera and both working normally again.

Did I forget to do something before taking the cards out of the Pad?

Cheers,
Peter