Excel stores the path of a linked workbook relative to the file that uses it, and rewrites it when the file is saved into another folder. The path was kept as text, so after Save As it named a file next to the new copy: a missing link, or another workbook with the same name. Relative externalLinkPath targets are now resolved against the folder the workbook was read from and written relative to the folder it is saved into (absolute when there is no relative path, on another drive).