It looks like the cmake rename command just doesn't attempt to support across filesystems. It either uses

rename

on POSIX ("The rename() system call causes the link named old to be renamed as new. If new exists, it is first removed. Both old and new [...] must reside on the same file system."), or

MoveFileExW

on Windows ("When moving a file, the destination can be on a different file system or volume. If the destination is on another drive, you must set the MOVEFILE_COPY_ALLOWED flag in dwFlags." <- this flag is not set).