Ole - have you checked the "Build Smart Previews" box checked when you import the images into Lightroom Classic? So far as I recall, unless you build these on import, you have to be physically connected to the drive to edit.
I've just imported a number of images from my external drive and have no issues editing in Lightroom after disconnecting the drive.
https://helpx.adobe.com/lightroom-cl...-previews.html
I personally don't like this workflow, as SmartPreviews take up a lot of hard drive space.