Thanks Paul- I think actually asked a similar question over a year ago- but never moved it. Now it's critical to get it off my Startup drive.Hi Jeff
You can, Lightroom is happy for it to be on an attached drive. Bear in mind the startup drive is usually one of the faster ones on a system, so ideally an SSD that's attached will work well. Having moved the catalog, just double click it and Lightroom will open. Be sure to then open it from the new location and not have the catalog duplicated and potentially opened by LR in both, or you'll lose changes!
One of the reasons for moving the Catalog to an external drive is space on computers primary drive, but another is to move LR (Cat & images) between a desktop and a laptop.
If you are sharing this Catalog between two or more machines, change the preferences Preset Tab under Location by adding a check mark next to “Store presets with this catalog”. The default keeps the Preset folders on the computer LR is installed on . If you do not make this change, when you switch the external drive to a laptop, your presets will not be available.
I also need to turn off my Smart Previews. I almost never work on the images offline and it's really bloated the size of my LR catalog!