pml you cant expect a solution with a description like that. what make is the HDD? how old is it?
IO error means it is failing to read or right files. it could be a bad cable, bad port on you pc, bad port on you External HDD, bad sector on the HDD etc etc.
start with giving some more specific info please. and try run a scandisk on the HDD to try find any potential bad sectors on the disk.
if you dont know how to run a scandisk, google it.