After linking two local Excel files by a simple reference to a cell in another workbook, I began seeing an ominous error:
SECURITY WARNING Links to external sources could be unsafe. If you trust the links, click Update. Click for more details.
This behavior was observed in version 1907 of Excel from the Office 365 software package.
Warnings of this nature should be taken seriously. In this case, however, the message has been seriously mislabeled. Clicking into the details brings up an ancient Help page for Excel 2007. Searching for similar situations online brings up some misleading instructions.
If you are experiencing the situation described above, continue reading below for a simple workaround and more background information.