So when you get the message "Destination Folder /Users/GlenE is not available", this usually means that the folder does not exist (or you have mis-typed the folder name); or possibly that you don't have write permission to this folder.
Make sure your External drive is attached and active on the desktop of your iMac before you boot Lightroom Classic. See the screen capture. You should then be able to locate it in the "destination" of the import dialog.
Regards, Denis: System iMac mid-2015, 5K 27” monitor, macOS10.15.6: LrC 9.4, Lr 3.4.1, Ps 21.2.3, Camera OM-D E-M1.
It is working now. The message locked up the whole screen. I could only force quit or click okay and have the fotos placed in the suggested location on main drive. So I clicked ok as it was only 10 fotos. That freed up the destination choices. Found the location I wanted to use and clicked import again. Voila, they are where they belong. Thanks for the help focusing me on the destination folder column. Would be nice if there was someway to cancel their suggested location and then find the one you want.