Move the collections from darktable (see [move] in [selected image(s)] module on right of the lightable. This is the safe way for dt to update the database accordingly.
When the external drive is connected you can access the pictures.
When the external drive is not connected you won’t have access to the pictures for development but the thumbnails will be visible on the lightable and you’ll still be able to add stars or keywords.
If you want to get full access to a collection (or a single picture) while moving away from your external drive you can use the [copy locally] action from the [selected image(s)] module. And when you are back you can [resync local copy].