Volume error on 20" C2D which TechTool won't repair

Question

My iMac was working absolutely fine last night, but when I went to boot up this morning, the blue "Starting Mac OSX" bar absolutely crawled across (it usually flies across) then, when it got to the right hand side, the boot hangs and won't go any further.
I tried zapping the PRAM - no change - so then I booted from the TechTool Deluxe CD. I ran all the tests except Surface Scan and it only failed on the Volume Structure test.
So I got TechTool to do the repair but, after a few seconds, it just defaults back to the entry "what language do you want to default to?" screen.
Running TechTool Deluxe again, it still fails the Volume Structure test and, when I try and boot the Mac, the same problem still occurs - it won't get past the blue "Starting Mc OSX" bit.
I haven't changed anything since last night, I haven't plugged any peripheral in or unplugged any.
I don't know what to do! Please help!
20" iMac C2D Mac OS X (10.4.9) 3Gb RAM, 500Mb HD, 2.33GHz processor

Answer 1

You will probably need a more industrial strength utility. I believe that you can upgrade to Tech Tool Pro. Many folks here in the Discussions swear by Disk Warrior. Some feel that if DW can't repair the problem, it cannot be repaired.
To be on the safe side, do you have important files on a back up?
Although you do not sound like the techy type I found this explanation of the issue;
http://computing.net/mac/wwwboard/forum/2584.html
<<To explain sibling links, first you need to
understand that most directory
information on Macintosh disks is stored
in data structures that are technically
known as B*-trees. Examples of B*-trees
on a Macintosh disk include the Catalog
File, the Extents Overflow File, the
Desktop Database, the Users & Groups
File, and several others.
Each tree has a number of data records;
for example, each record within the
Catalog tree describes one file or one
folder. The records are sorted and
indexed for fast searching. But for
efficiency, the records are also grouped
into fixed-length blocks called nodes;
because these nodes can be physically
scattered around the B*-tree file, they
need pointers to indicate their logical
ordering. So a given node has a "forward
link", which points to the node containing
records that are logically immediately
after the records in the current node;
likewise, a node has a "backward link",
pointing to the node containing logically
preceding records.
If these linkages are pointing to the wrong
place, some files or folders might be
inaccessible. Or the OS could even hang
up trying to find some. This makes the
link invalid.
In any case, Norton Disk Doctor reports
forward link and backward link problems
separately, whereas Disk First Aid just
refers to them collectively as sibling links.
Try DiskWarrior to build a new directory.
May fix the problem without having to
initialize the disk.