Lightroom produces image "previews" that are stored in a separate folder than the RAW data. Most of the time, these previews are what you're actually looking at because they're much faster to render edits. You can specify in preferences what quality you want these previews to be, and where they're stored.
Based on what you've told me, I'd import the RAWs directly to the external HDD and render the previews on your local laptop. If you're in the field and have to import the RAWs to your laptop, just make sure you use LR to move the files once you're home, so LR knows where you've put them.
With the previews on your laptop, you'll be able to do a considerable amount of editing even without the original file available because your edits are basically a set of rendering instructions and they can be applied to the low quality preview as well as they can to the original RAW. You'll just see a little icon over the preview saying that LR can't locate the original file until you've plugged in the externals again.
You won't, however, be able to export your work in full quality until you're back in the office, and the results will probably be slightly different, but not enough that you'd care or even notice if you're not using carefully calibrated monitors and printers.