Lightroom 5 has completely changed how you actually work in this disconnected mode, so I will explain how to do this in LR5, then how to do it in LR4 and earlier, for those that do not have LR5 and later.

Lightroom 5

Lightroom 5 has added a function called 'Smart Preview' that transforms the needed workflow in earlier versions of the software. Due to this, you can afford to leverage ONE catalog. In this case, you will store all the images on the external hard drive, but the LR catalog on your laptop. Smart Previews allow you full editing capability, even when the original image is not available.

Existing images:

• For all photos you wish to be able to edit, you simply ask LR to render Smart Previews.
  This way, you can edit when disconnected from the external hard drive, and when you reconnect it, LR sync's the edits. This is available as a shortcut from the histogram within the Develop Module, and within the Catalog dialogs.

Adding new images:

• When you add new images when disconnected from the external hard drive, LR will have you copy those images to the laptop hard drive. Edit as normal.

• When you reconnect to the external hard drive, you need to DRAG those new images from your laptop to the external drive from the Library Module.

• If you create Smart Previews when importing, you can edit when disconnected later.

That is it: LR5 has made this dead simple. There are some videos on the Adobe site that explain Smart Previews in great detail.

Lightroom 4 or older

One downside to LR 4 is that there is no equivalent of Smart Previews. Unless you duplicate images to the laptop, you will not be able to edit them when disconnected from the hard drive. However, keeping your catalog on the laptop will let you easily import new images when disconnected, and edit them on the road.

Keep your catalog on the laptop.

• When you add new images when disconnected from the external hard
  drive, LR will have you copy those images to the laptop hard drive.
  Edit as normal.

• When you reconnect to the external hard drive, you need to DRAG those new images from your laptop to the external drive from the LIBRARY MODULE. Don't do this from Explorer (Finder).

Catalog on external drive (or two computers sharing images)

If you want to have a selection of images available for editing when disconnected, then you should take advantage of LR's 'Export as Catalog' function.

Essentially, you will be using two catalogs: one on the laptop, and another on the external hard drive. Lets call the one on the hard drive your 'primary' catalog, as it will be the catalog that contains all your images. We will call the catalog on your laptop your 'traveling' catalog. Personally, I prefer to keep the catalog on my laptop hard drive, while the external hard drive contains all the images. If you do not have room for this, you can safely store your catalog on the external hard drive. To avoid confusion, I won't refer to the location of the catalog, only the defined name.

Primary catalog

First, import all your images into the primary catalog. This is when you will store all your photos and will become the 'canonical' catalog.

Traveling Catalog

When you are on the road, away from your external hard drive, you no longer have access to the images in the Primary Catalog. You may also not have access to the actual Primary catalog (esp true if you use TWO computers). In this case, you, or LR will create a second catalog, we are referring to as Traveling catalog.

When on the road, you will need to use the Traveling catalog. Open LR, and choose File > Open Catalog, then choose Traveling Catalog. This way, when you take new images, LR will import them into the Traveling Catalog. Since the images are local, you can edit as normal.

When you reconnect to the external hard drive, open the Primary Catalog, Next, File > Import from catalog, then choose your Traveling Catalog. This will import all the images and edits into the Primary Catalog.

If you wish to be able to edit a selection of images from your Primary Catalog when disconnected from the external hard drive, then you need to import those images to the Traveling Catalog. From the Primary Catalog, select the images that you want to transfer to the Traveling Catalog, select File > Export as Catalog, be sure to check the box called "export negative files", as this will also copy the RAW files. Name the catalog some other name, as this is a temporary file. Then open Traveling Catalog, File > Import from catalog you just created. When done, delete that temporary catalog. This action copies the RAW files and all edits to the Traveling catalog. If you make edits, you can sync them later by Importing within the Primary catalog.