what does an error code 10657

So I initially got a message that there was not enough memory on my start up disk and that I should close applications that aren't being used. Check the memory on the hard drive and found that I have used like 74.01 GB of 74.2 GB. So I got a hard drive and did a back up to it. Now when I try and open an application off the hard drive, like iphoto, it gives me the message "an unexpected error has occured... error code -10657" What do I need to do to open space on my HD and to make it work off my external hard drive. Please keep it simple, my knowledge of working with computers is very basic. Thanks.