Thank you Asala42!
I've had a broken link for about 2-3 years and I've searched through every page trying to find it and no luck several times over the years, so I've just dealt with it. It must just be a Layman thing (at least phonetically), as that's my last name.
Anyway, I created a copy of the file and then I went through what you said below and had no luck, including pointing the link back to the same spreadsheet (but it failed), so I just started deleting a single sheet at a time, saving the workbook, totally closing out of XL and reopening the workbook again and again until the broken link didn't appear when I opened the file. Then I went back to the offending sheet on the real workbook and I don't know why I tried this first, but the first thing I looked at was the Data Validation (since you said it was the most rare, it had to be it, right?), and the first Data Validation drop down menu reference for some reason was linked to a temporary file on the C: drive which has been reimaged about 6 times since the link was first created.
Thanks so much...this has been annoying me for a while.