Hard links do not use up inodes.

Here's the mostly complete introduction:

When you see the word "inode", just think of it as being another name for "the file data stored on disk". Here's how it works:

Each file/directory on a filesystem uses one inode -- a data structure on disk which describes the file location/size/etc. Inodes themselves are not useful until they are linked into a directory somewhere, so that applications can access them. The process of linking an inode into a directory, and thereby giving it a "name", is known as hard-linking.

All files are "hard-linked" to at least one directory when created. Additional "hard links" can also be created, which merely cause the same inode ("file") to appear in more than one directory. No new inodes are used for this.

But since directory entries reference "inodes" within the same filesystem, and only within the same filesystem, the inodes MUST be on the same filesystem as the directories which link to them.

So a file ("inode") on the /drive0 filesystem can only appear in directories on the /drive0 filesystem.

To complete the story, there are also "symbolic links". A symbolic link is not really a link, but rather a way of storing an arbitrary string in a directory entry. This creates a directory entry which does NOT link to any inode ("file"), but rather just contains a "target" string, which is re-interpreted at access time as a new path to look up.

When accessing the target of a symbolic link, the kernel just grabs the string from the directory entry, and then does another lookup search using the new string for the path instead of the original string that referenced the symbolic link entry.

Since symbolic links are just strings, they can contain anything, including "invalid" strings that don't contain a valid pathname (in other words, they don't have to reference something that actually exists). And because they are just strings, they are not restricted in any way as to where the so-called "target" file resides -- so they can be used to cross filesystem boundaries.

Cheers