this problem due to a virus that change file attributes "autorun.inf". so we need to change back this attributes to make these files visible again. but before doing that we have to remove autorun.inf virus.

1-run a scan using auto-run eater to delete autorun virus: http://oldmcdonald.wordpress.com/

2- Open the command Prompt Cmd and type this command bellow to make your data visible(x= external hdd letter):
attrib -H -S x:\*.* /S /D