If you have a corrupted PCMCIA card that your Newton MessagePad that won't recognize follow these steps:
1) Remove the PCMCIA card from your MessagePad and restart the MessagePad.
2) Tap the Extras button, then tap Prefs.
3) Insert the corrupted PCMCIA card into your MessagePad. A dialog will appear which asks if you want to erase your card. Tap Yes.
Your card should now work correctly.