Today’s Question: You say you keep your Lightroom catalog on you laptop but your photos on an external drive and this makes sense if the laptop is your primary computer. Would you not want your catalog and photos together on an external hard drive when using a laptop for travel but then transfer the catalog with photos to a desktop when you get home? Otherwise would you not need to move the catalog to the external drive every time you want to transfer to a desktop?
Tim’s Quick Answer: In my case the decision is easy, because my laptop isn’t merely my primary computer but rather my exclusive computer for working with my Lightroom catalog. For those who need to work across two computers, keeping the Lightroom catalog on an external hard drive can be a reasonably good solution, though it is important to keep in mind that this approach will generally lead to a degradation in performance for Lightroom.
More Detail: One of the challenges many photographers face when using Lightroom is that there isn’t an easy solution for working with a single catalog across more than one computer. You can certainly copy the catalog back and forth across multiple computers, but to me this is a very risky approach because you need to keep track of which copy of your catalog is truly the latest version (and be sure not to use an outdated version).
Another approach is to keep the catalog on an external hard drive. However, as noted above, this can in degraded performance in Lightroom. In some cases this degradation in performance can be significant.
Some photographers employ a solution such as DropBox to synchronize a catalog across multiple computers. However, this approach makes me nervous, because connectivity issues can cause synchronization to fail in problematic ways.
As a result, to me the best solution is to keep Lightroom on a single computer if at all possible. That, in turn, is part of the reason I have opted to use a laptop as my primary computer, including for my Lightroom-based workflow.