I had a similar problem. Got a card with my Gold System and it keeps telling me the card is "write protected". Trying to reformat it does nothing, so based on what others have told me and my own experiences, it was assumed that the card's batteries are dead.
Have you tried to reformat the card? Put the card in and power up the Neo with a game cart in the slot. Then hold down D on controller 1 and hit "reset" on your console. Try to reformat it using option 1. If it won't let you reformat or says the card is write protected, then I think your card is dead.
I've been looking for a certified working memory card (I don't care if it's new or used...must be tested and working)for a few days now with very few offers. Maybe Playmore should consider making or selling some Neo compatible memory cards as it seems the current stock is becoming unreliable...