PS2 mem card probs

[taffio]

Hi, can anyone help i've got a chipped ps2 and a back up of san andreas which my son plays ..................a lot, The problem is his friend has the same but is a bit further on in the game (got more jets , helicopters that sort of thing)which my son wanted so he decided to take his memory card over and then save onto our memory card. But instead of copying the the game over the proper way they decided to swap memory cards over during gameplay . now it says corrupted data and we can't delete it nor can we save over.so it basically says we can't delete or save the game onto that memory card.

any ideas plz my son is doing my head in

thx in advance