I have a macbook pro with only 128gb of hard drive. I have lightroom installed and all my photos on an external hard drive. Can I store my lightroom library on the external hard drive too?
Yes. Find the LR catalog file by looking in Edit/Catalog Settings/General. Close LR and navigate to that spot. The catalog file is the one that ends in .lrcat - move it to wherever you want. Next to the .lrcat file is a subfolder called Previews.lrdata. You can move that over as well, but if you don't LR will generate new previews. Double-click on the .lrcat file in its new home and LR will open with that catalog. Set Preferences to When Starting Up Use Most Recent Catalog.
Thank you for your help. I got everything to work.