Do current O/S's not give me an icon or notice of some kind indicating that I've plugged an external USB drive into my Macbook Pro?
There is a very different learning curve with Mac O/S that's not so intuitive in operation. If I can't "see" my attached external USB drive with my music library on it, I won't be able to import the library to MC 20.
There is definitely a learning curve with Mac OSX. Like anything new, it's going to require some effort at first. Once you "get it", you should find it pretty easy and intuitive. The funny thing about OSX, for people that have used Windows for a long time, is they tend to overthink how to do something. Years ago, when several of my technical colleagues and I switched to OSX, ,we would all laugh at each other for the OBVIOUS things in OSX that we went searching for.
Let's address your question specifically. All external drives are mounted on OSX when you plug them in. I say "mounted" because they won't show up as a drive letter; that's a windows thing. Instead, they will show up under Devices in Finder. Finder is the file browser with the blue happy face icon on the Dock.
If you don't see your device in Finder, follow these simple directions to make it appear:
https://support.apple.com/kb/PH19062?locale=en_US&viewlocale=en_USYou have said it's a WD for Mac, so it *should* be natively Mac formatted. If it is, you should be good to go. But that begs the question: If it's natively Mac formatted and you have music on it, how did you get music on it in the first place? Windows won't read the drive because it's for Mac. Maybe you copied music files from another Mac?
If instead it's formatted for Windows as NTFS, you'll need to reformat it. I'll leave that for another discussion though.
Brian.