Appear to have noticed that there is a difference in the location that an excel called DLL wants any utilized data (.dat) files based upon where the DLL itself is stored. For example, when the DLL is stored in C:\Windows\System the files appear to be sought in C:\Users\Owner\Documents. When the DLL is stored in SYSWOW it looks like the .DAT files can be in the same folder as the spreadsheet. Is it understood how this works? What are the associated rules around this?
This is probably not specific to Fortran.
It's likely related to what happens to be "current directory" when the data files are opened. See also this link: https://stackoverflow.com/questions/971296/how-to-use-dlls-in-the-same-directory-as-an-excel-file
Where the DLL is makes no difference whatsoever in where data files are looked for.