Months after the release of macOS 10.13 High Sierra, folks are still having problems with limitations of the new Apple File System (APFS) format required for SSDs that run High Sierra, and which you can optionally upgrade other drives to use. The archive becomes useless, because APFS doesn’t support hard links. These are a special kind of alias. A soft link is a pointer to a destination file that looks to the operating system like a pointer. A hard link looks to the operating system like an actual file, even though it’s just a pointer. This allows a single copy of a file to be in a filesystem, but have many pointers that reference it, and they can be manipulated and copied as if they exist in multiple places.