I have recently moved my LR catalogue to an external HD and started to keep images on this drive. Previously if I imported images into LR from an SD card onto my internal drive, it recognised that theses images had been imported when I added new images to the card and imported them :e.g. all previously imported images were grayed out on the import screen.
Now when I import images from the SD card onto my HD, the import dialog box always shows all the images, including ones previously imported. Note that the import process includes renaming the images. But I have always done this in the past anyway.
Where is the Destination location set to?
Are you still using the same Catalog, IE the one you moved to the external drive?
Please show a screen shot of the import dialog with SD card in the slot with new and old images on it. Expand all sections in the right panel.
Sorry folks. I should have spotted that "don't import suspected duplicates" had become un-ticked. That was the problem as i always have kept that selected in the past.