Why no image preview for raw images on external hard drive?

Ok, this is one of those "there must be an obvious answer I am overlooking" issues. I returned from an overseas trip with many gigabytes of RAW images, which I backed up directly from the flash memory cards onto an external portable hard drive. I would like to select images from this hard drive to import to Adobe Bridge/Photoshop CC and on a different computer, Photoshop Elements 2018. When I navigate from the Adobe programs to the hard drive to import files, these appear as generic icons with little mountains on them and the file number. In the "get photos and videos" dialog box there is an option for "generate previews" which is grayed out.

I would like to be able to see thumbnails of the images on the hard drive so I can decide whether or not to import them to the program I will use to do the postprocessing. Do I need to import them all into the Adobe program and then send them all back to the hard drive? Advice welcomed.