Hard links are nothing very special. They are actually really simple. The first thing to understand is, that a file is not identified by its path and filename, but simply by a unique number - the id of the “inode”. Every file in the filesystem has such an id independent from the filename or the path it is in. But where does the filename come from? A directory is by itself just identified through the id of its inode. You can imagine a directory as a special file that contains a list of filename to inode mappings. And this is how hard links work: you can have multiple entries on one directory or in different directories who map to the same inode (file or directory). Since inodes are only unique within one volume, the hard links only work within a volume. If you copy or move a hard linked file to another volume - it will be a copy. Actually copying a hard linked file is nothing special at all - you could imagine any file as being “hard linked” here is no difference between a hard linked file or a normal file it is stored the same. It is just the same inode id used another time. A symbolic link works different - the filesystem stores the pathname of the file and identifies it as a symbolic link - every access to this link gets resolved to the then current inode at his pathname. This way symbolic links also work over volume boundaries. So if you delete you Aperture library, anything that happens is you deleting some directory entries. Since the Inodes of the RAW files are still in the Photos directory entries they are still available. If you delete the Photos library too, the inodes are still available, but the space can get reused any time later. You could reconstruct a directory from those “lost inodes” with a rescue tool like Disk Warrior.

Jochen is right-on-the-money. Files are just blocks of space on disk. An inode points to the first block. Large files consume multiple blocks that are chained together and may be spread out all over your disk. Does anyone remember defragmenting Windows disks? That basically reshuffled all the blocks associated with files so that all the blocks for a single file were as close together as possible.

Files are pointers / mappings of file names to inodes. A directory is a special file that is a lost of these mappings. You can have multiple “files” map to the same inode. These are your hard links. The filesystem keeps track of how many mappings there are. Only when the last mapping is deleted does the actual space on disk get freed up. So if Aperture and Photos have separate mappings (hard links) to the exact same inodes on disk and you choose to delete your Aperture library, the filesystem knows there is still one mapping from the Photos library and so it keeps the inode and blocks of storage active. If you later delete the Photos library, the filesystem will see that this is the only mapping to the disk blocks and thus will free up the entire chain of blocks associated with all the files in the Photos library.

This web page gives a visual that helps illustrate soft links vs hard links.

http://www.geekride.com/hard-link-vs-soft-link/

The Lightroom importer has an Option screen where you can choose to leave your originals in their current location. By default, Lightroom will make a copy of all of your images and thus consume twice the space. If you choose the option to leave images in their current location, Lightroom will reference the same copy as Aperture. All Lightroom catalogs are Referenced (as opposed to Managed) so it is wise to relocate your originals outside your Aperture library before using the Lightroom importer unless you choose to let Lightroom make a second copy of your images.

Capture One doesn't give you a choice. It defaults to referencing the Aperture originals in their current location, regardless of whether your library is Managed or Referenced. If you import a managed library into Capture One, Capture One will refer to the originals where they live inside the Aperture library. As with Lightroom it is recommended to relocate your originals outside the library (move Managed images to a Referenced location) before using the Capture One importer.

The difference between Capture One Catalogs and Lightroom Catalogs is that Capture One supports both Referenced and “Stored” Catalogs (“Managed” in Aperture). Lightroom only supports Referenced Catalogs.

[ UPDATE ] Now that I have upgraded to Lightroom CC (Lightroom 6), the Options screen has more clarification about leaving originals in their current location. Managed images will still be copied. The Option screen explicitly says to select the option to leave REFERENCED images in their current location, and that Managed files will still be copied. This is a very important clarification if you are using the Lightroom importer to import Aperture libraries. Look at the last section of this screenshot.