Why aren't my external hard drives showing up?

I just picked up my computer from getting fixed due to an "imminent hard drive failure".

I backed up my files on my external hard drives before taking it in. Now they aren't showing up. How can I fix it?


EDIT: btw, when I plug them in, I hear the little ping that a usb/ drive was plugged in and I hear it spinning, but it is not showing up.


EDIT PROBLEM FIXED! I HAD TO DOWNLOAD A COMPATIBLE DRIVER