External USB 3.0 Seagate drives no longer mounting?

in macOS
Has anyone run into an issue where their external USB 3.0 hard-drives will no longer mount with macOS High Sierra? I have just run into this and suspect it is High Sierra related, since they still mount in another Mac running El Capitan. The odd thing is that they don't even get recognised by the OS anymore.

I am going to try to create a MacOS 10.12 recovery disk, just to make sure that my conclusion is right.
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