Move 1 TB of previously imported files to external HD?

I think it's recommended that one keep image files on the same system HD as the catalog

By @default9glollkfs9z6

That’s only how it works by default, but Lightroom Classic has the tools to manage originals on any number of internal or external volumes. So it is totally OK to do what you are saying. If you add photos from different volumes to a catalog, each volume is listed separately at the top level of the Folders panel (I have several drives listed there for my Mac). In your case, if you are moving the entire folder set of originals from an internal to an external volume, after you finish moving and relinking, the top level of your Folders panel will now show the external volume at the top level.

I think it's recommended that one keep image files on the same system HD as the catalog

Not recommended at all.

Do you think it's OK to do that?

Yes

Follow these instructions, move the photos in your operating system, keeping folder structure and location of the photos within the folders unchanged, then see Figure 4.