my MacBook air doesn't recognize the external drive

Hello! i have a problem with my external drive. i wanted to make air time machine at the first time my computer recognized him, but his not allows me to make a backup. now i see the external drive, but just in the disk utilities. in finder i can't see it.
I think to replace him, but i want to know if there is something to do before? i had activated the first help in order to try fix that, but i get a message that its fail. I had try time but no success. thanks for your answers.