MacBook - error message when trying to open external hard drive files

so i recently had the hard drive on my 13" macbook replaced. i used a 160g hitachi external hard drive to back-up previously. when i open the external hard drive file it only allows me access to a few pictures and it saved my itunes from months ago (none of the new stuff added to itunes is accessible), when i go to open a file from a specific date on the hard drive i get an error message.... please help!!!!