Copying The Lightroom Folder

Expanding on this, let's say your current Catalog has imported some images stored on your internal C: drive, and also some images stored on this external drive (D:, for example). Moving the catalog's storage location will not alter all those remembered drive and folder paths that it holds for imported images. 

Recommendation 1: assign this portable drive a fixed drive letter - one far enough through the alphabet to not risk ever coming into conflict with a drive letter already taken up by something else, So not D or E - better if it's say P for "portable" - whatever you like. MacOS only cares about the drive name, but Windows needs more than that.

Recommendation 2: consider (if you have not already done this) having all the imported image files within this same external drive as the Catalog. Otherwise a fileshare within a NAS or server, may work well across a local network (though that's no good for the Catalog itself).

If bulk moving / copying image files between drives, you do then need to inform the Catalog of their change of location. This just involves readdressing the top level of that entire tree of folders..This action is also necessary, any time the drive letter changes for whatever reason. Which is why it's a good idea to fix that.