My .lrcat is located on my MacBook hard drive (first picture). My pictures are accessed on an external hard drive when I use Lr (second picture):
I want to edit pictures on both my MacBook and my iMac, using the same catalog. Do I move the catalog from my MacBook HDD to my external drive? And if I do, what do I need to do to make sure both machines access the same catalog (one at a time, of course)?
Yes, you use your operating system to move the catalog file to the external HD, then double-click on the catalog file to open it.