If it is not the Pro version, then it is probably faulty, I havent seen the standard Duo version around for a while... Did you get an original memory card with the phone? If the original doesnt work then it is probably a fault with the memory card reader on the handset, if you can try another card, see if you can format it and use it. The P910 can use either Duo Pro or standard Duo, but the P900 can only use the old Duo, I think there was a limit of 128 on this as well but I cant remember
You could flash the P900 with P910 firmware