I am currently using a 2GB SD card in my 50g without any problems.
Re-format the card, you will have to use a PC for this as the 50g cannot format 2GB cards. Just format is as default FAT. (I'm sure mine is FAT32 which works fine). Remember to back up your programs first.
Try a different SD card
Ensure all calculator batteries are good (including CR2032 RAM backup battery).
Reset the calculator.
Press ON and F3 together. Calculator will reboot (does not delete data or change settings)
Hard reset #1:
THIS WILL SET DEFAULT SETTINGS AND DELETE ALL DATA (except port 2 flash).
Press and hold ON then F1 and F6 (i.e. hold all 3). Release in reverse order.
Hard reset #2:
insert a straightened paper clip into the reset hole on the back (this could also delete data and set default settings).