There is a very simple answer; you should have used an internal hard drive.
The problem with an usb external hard drive is that they can course problems with your computer; they are slow and take up too much of the resources.
I don’t know how old your computer is, but you would be better off with a mac for this kind of thing.
Try booting your computer with the new drive plug in and run a scan on it.
Make sure its formatted properly and there is no “funny” music tracts with build’t in ant-copy protection.