So it shows the link as missing.
As you say, that doesn't prevent you, however, from doing edits and saving them, but the updated link is not "functionable"; you'd have to save a copy of the SU file to your PC or a server, then re-link to that.
I was kind of surprised by this. After creating a Layout file, I deleted my original SKP fle. When I opened the Layout file and edited the viewport with SU, my link path was identical that which you have shown. It is interesting to me though, that when I did a File > Save As or File > Save a Copy As that an "external" file was created in my Document and Settings path [see below].
I would have thought the process of updating the invalid link in Layout would have automatically searched this location and revised itself and shown that path.
SaveAs.png