Answer:
If the problem is with an external hard drive rather than your computers internal hard-drive then you don't need this guide, just hit repair.
This is for if you have run the disk utility on Mac and it has come up with a problem and said you need the installation CD and you don't have it to hand or have lost it.
Basically you just need to not be using the operating system while you are repairing it, hence the loading from disk, or as I will show you - boot into the terminal or mac equivalent of the DOS screen.
Please ensure all files are backed up and a copy of the Snow leopard or whatever operating system is important to have first. Prevention is better than cure. So keep your files you need backed up before doing this though.
Print these instructions off and notice where I have put spaces as they are important.
1) Restart your computer.
2) As soon as the screen has gone off and before the chime hold down "CMD" and "S" to boot into single user mode. A screen like terminal should load and end with root.
3) Type the following command on the separate line below without the speech marks and (notice spaces between "hfs" and "-" and "r" and "/".
"fsck_hfs -r /dev/disk052"
The disk will then do the repair it would be doing if you had the installation CD in, as the disk is not being used by the operating system and so can be checked and repaired. This can take 10 minutes or so.
Once it finishes doing its checks you will be prompted you can type again with the "root" text shown again.
Now do a disk check by typing the following (again notice space between the "fsck" and "-fy")
"fsck -fy"
If it comes up Disk is Ok, all is good.
Then type "reboot"
and computer will reboot. Check disk in disk utility and should say "disk appears ok."
If it doesn't repeat the whole process again, or see below.
If these fail please try applejack (free) or Disk Warrior (paid) programs. If these fail, then it might not be software and it might be the hardware is a problem and you will need to contact apple or a computer repair shop to fix the problem.
Source: http://mahalkita.nanogeex.com/2007/09/28/how-to-fix-the-invalid-sibling-link-error/ showing this worked for many other people and it worked for me on my macbook pro running snow leopard.