Same start up problem as previous posts, on Powerbook G4-Ox 10.4. Gray background with Flashing two faced mac icon and "?" and it stops on "?" after a while.
I used Disk utility to repair HD and gave me this:

Invalid node structure
Rebuilding Catalog B-Tree
The volume Macintosh HD could not be repaired.

Error: The underlying task reported failure on exit

So what now???How do I save any files prior to erasing and re-installing Tiger.

DiskWarrior, Drive Genius, TechTool Pro or TechTool Deluxe that comes with AppleCare can all repair that problem. However, If you do not wish to invest $80 to $100 in one of these utilities then yes, backup everything you can and then boot from the install DVD to run Disk Utility and erase/reformat your hard drive.

Under the circumstances I would feel better if I ran a surface scan on the drive just to be certain the drive itself is not failing. A drive with new bad data blocks is 39 times more likely to fail in the next sixty days than a drive with no new bad data blocks and is the best predictor of impending drive failure. Significantly more accurate than the S.M.A.R.T. values reported by the drive itself. Drive Genius or TechTool can do that for you.

If you have the TechTool Deluxe program that comes with AppleCare, run the Surface Scan. In TechTool Deluxe, the Surface Scan is limited to the internal hard drive, but that will serve your needs.

I would get rid of my wife before I got rid of Disc
Warrior. A must have for any half way serious Mac user.

I like to think I'm a "half way serious Mac user," and I've never used DiskWarrior.

(Of course, I've never equated my wife with a software utility, either.)

Just to enlighten you a bit.
http://www.math.unipd.it/~favero/varie/ragazzauk.html

Neat!

Still need assistance? Follow the yellow brick road!