Try reformatting the memory card.
Use a different drive to read the card.
Run this software:
USB Flash drive tester
Flash card/drive tester allows testing of any removable media including SD, MMC, CF, USB flash pen drives for bad or unstable sectors. Especially useful for testing for fake sizes often seen on low quality USB pen driveshttp://downloads.zdnet.co.uk/0,1000000375,39364218s,00.htm