I am running out of hard drive space which is the source for importing my images into Lightroom 4.3. I have these images backed up on an external drive. I now want to use the external drive as my source of import. How do I do that. And then how to do I find the missing files in Lightroom?
I think there may be a problem with your nomenclature.
You want Lightroom to start using (pointing to) the images on the EHD rather than the Internal?
You want to import all future images not yet in LR from your EHD?
You will need to repoint the folders in the Folder's panel to the EHD.
This works best if you photos are in folders that all nest under a single or just a few Parent folders. Then you can simply repoint the top level folder and all the subfolders will follow. If not, you will either have to expose a hidden Parent folder (if one exists) or repoint each folder in the Folders panel individually.
If you can see 2012 but not Photographs in this example you can Right/CTRL Click on the Folder and tell Lightroom to Show Parent Folder.
Go to the Folders panel
Select a Folder
Right Click (PC) or CTRL Click (Mac) on the folder icon.
Choose Update Folder Location…
Navigate to the folder's location on the EHD
Press [Select Folder]
Repeat as necesary.
Test one or two folders. You can always point them back if isn't working well.