'Not enough data to compute'
When you have a computerproblem you should tell us:
-OS + version
-used memory card
Do you have the drivers of the cardreader installed?
I never had any problems with reading a memorycard from a camera (most are FAT32, so they are compatible with both windows and apple), so maybe the cardreader is just malfunctioning or not installed properly. Or the cable to your external cardreader is damaged?
Are you sure the memorycard is placed correctly?
Are you sure the memorycard is alright?
(Just a few guesses you could try to eliminate.)