Previews disappear when I move photos to SD on Chromebook

[#1]

I have a relatively large micro SD inserted into my Chromebook for memory expansion.  Lightroom suggests that I move my photos there.  I would love to be able to do that, but when I do, Lightroom doesn't render any previews.  They are all gray in the grid view.  They are fine when I select an individual image, at random of course because I cannot see them. 

If I return the photos to internal memory, then the previews show up fine.  I specifically have the SD card to unload the internal memory.  It would be great to be able to use it...

[#2]

I have a relatively large micro SD inserted into my Chromebook for memory expansion.  Lightroom suggests that I move my photos there.  I would love to be able to do that, but when I do, Lightroom doesn't render any previews.  They are all gray in the grid view.  They are fine when I select an individual image, at random of course because I cannot see them. 

If I return the photos to internal memory, then the previews show up fine.  I specifically have the SD card to unload the internal memory.  It would be great to be able to use it...