For my workflow, I copy from the camera SD card to an external HDD, then import to Darktable from there. It stores its thumbnails and database under my home directory, and I can ensure it has thumbnails and medium-resolution previews of all photos with `darktable-generate-cache`. On any photo, I can click "copy locally" [1], and it syncs it to my internal SSD so I can edit without the HDD plugged in.

[1] http://www.darktable.org/usermanual/en/local_copies.html