Catalog Location when working with Multiple External Hardrives

Provide some additional info of your computer resources, this will help others to provide options.

Internal disks, external disks, the capacity of each disk and the free disk space available.

Its, best to have your Catalog file and previews on the fastest drive, internal if possible, the actual image files can reside on the externally connected drives. Note the Catalog file does not contain image files or copies of the files, it's just a data-based file with information.

E.G. Internal SSD drive 1TB, OS, Applications, LrC Catalog-Previews and Smart-Previews etc. > External disk 1, 3TB original image files, External disk2, 3TB original image files> additional backup facilities NAS. This would allow you to use the Smart-Previews for editing if the external disks are not connected.

How often do you work with images from both external drives at the same time?

If not often, copy the catalog and its suppporting folders to the drive that has the images you are using most of the time (assuming the new ones).

Since you have original files on both hard drives, remember that you need separate hard drives as backups for those old drives. In your case, I might get a large HD, copy the images and catalog to the new drive and use the old drives as the backups.

I'm using an old HP Pavilion desktop that I have modified. I have a SSD as the main internal drive, two other internal hard drives and two external hard drives. LrC and the catalog are both installed on the main SSD. My images are distributed across the external or internal hard drives, but are managed very well by the catalog on the SSD.

There’s no need to create multiple catalogs, since a single Lightroom Classic catalog can track images on any number of external volumes (each volume is listed in the Folders panel). Just track everything in one catalog.

For moving among multiple Macs, the simplest solution is going to be that one big volume with both the one catalog and all the images.

The alternative, Lightroom (not Classic), stores all images in the cloud so they are always accessble from any desktop or mobile device that can run Lightroom. But if you need the features of Classic and you don’t like the idea of needing an Internet connection to work, then you’re back to the one big volume to carry around.

Depending on your budget and the storage in TB that all of your images require, you could save space and avoid packing an AC adapter by carrying around a 2.5" laptop-sized volume, or two of those in a portable multi-bay enclosure. Or a Samsung T5 or X5 SSD, which are both faster and smaller than a 2.5" hard drive. Or an even slimmer NVMe SSD available up to 4TB (for a price), in an enclosure only half as big as a smartphone.

Theer is no easy way.

Possibly buy a very large external drive and place all images and the catalog on that very big external.