You need to provide more information.
What brand, model & capacity is the hard drive?
Did you previously have this ext HD mounted on any Mac running OS X?
Is the ext HD connected using USB, FW400 or FW800?
Has it ever been mounted while connected to this MacBook?
In the sidebar does it appear under Devices or Shared?
Have you looked at your permissions for the ext HD? Are you even able to get info about the ext HD?
Are you logged in using an Administrator account?
i'm working with a macbook and I'm having trouble with my external hard drive.
when i connect it, it does not show up on the desktop. it does appear in the side bar of disk utility but it says it is still unmounted and will not let me mount it.
i cannot verify or repair the disk in first aid. nor can i create a new disk image onto another drive.
this has never happened to me before so i am very lost. i've googled my brains out but haven't really found any answers to my specific problem.
this drive has everything important on it so i would really appreciate anything that could possibly help.