Using "dd" restore an unreadable volume: Help!

First of all you should find out if the drive has a hardware problem.
Since the problem with the drive could also be related to a defective FW bridge, which resides in the case of the drive, you should take it out.
Disassemble the case, set the jumpers at the drive to "slave" and mount it inside your G4. There is a problem however, that your machine probably does not support ATA drives larger then 120GB. Check that out, and if true, you have to search for someone owning a newer G4.
find out, about the diskname:
sudo pdisk --list
you should be able to identify your disks name, either rdisk0 rdisk1 rdisk2 ...
lets assume its rdisk2
Then, do a:
sudo dd if=/dev/rdisk2 of=/dev/null bs=10485760 conv=noerror,sync
and wait for a long time.
If the process finishes without an error message, the disk should be ok. If you get a message about error reading, the disk is bad.
copying the disk with DD will result in black holes of zeroes where the data is unreadable.
If you get no errors, buy a copy of disk warrior, and give it a try on the drive.

If you dont want to take the drive out, at least do a:
sudo dd if=/dev/rdisk2 of=/dev/null bs=10485760 conv=noerror,sync
even if the drive is inside the FW case.
Trying to read bad sectors while the drive is inside the FW case might probably give you no error messages. Instead, the whole machine will hang.
However, if the process finishes without error messages, try Disk warrior on the drive.

Assuming, the disk is bad, but has not more then about 100 bad sectors:
Chances are good you can recover all data:
Go to someone who has a recent PC-compatible.
Buy a copy of HDD Regenerator ( www.dposoft.net)
connect the drive to that PC, run HDD regenerator. If this is able to repair the drive, put it back in your Mac or FW case, and immediately copy all your important data to another disk. Then replace the drive.
Most of the time, HDD regen works very good. However in rare cases it might worse the situation, rendering the drive totally inaccessible ( especially when you have a western digital drive with many bad sectors)

If you do not know someone with a PC compatible:
Get another disk size 200GB or more, and copy the disk:
sudo dd if=/dev/rdisk2 of=/dev/(Raw disk name of new disk) bs=512 conv=noerror,sync
And wait for a very very long time!

After that, try to run diskwarrior on the copied disk. If successful, copy all needed documents from the repaired disk to somwhere else and reinitialize the Disk. Some Documents might be unsusuable since DD replaces missing data with zeroes!

This is a very short Howto. However, I hope it helps a little.
If your data is very valueable to you and you dont have a backup, consider sending the drive to someone who is used doing data recovery.

no much to add, but if the data on the questionable drive is valuable. Before attempting to repair that data. I would recommend running the dd as Nobody suggested, saving the disk image to a file. Then assuming that you have another drive that's large enough, a scratch disk. use dd to write the disk image to the scratch disk. Then apply your recovery efforts on the scratch disk. Doing it this way allows you try over and over again, if necessary. It also allows you to try different techniques. By doing it this way, if you make a mistake, you haven't lost anything. All you need to do copy the image file back on the the scratch disk and start over again.

The rdisk device is the "raw" disk device - if you like, look it up in the apple dev pages. DD is unlikely to operate on the "disk" device.

I suggest the larger block size to increase the speed of the copy. If really want to copy your disk and not only check for defects, I would use a block size of 512. You can use a larger blocksize even if you copy the disk, but I think you might get less detailed error messages about the unreadable blocks.

Disk Warrior will repair filesystem defects. If your disk has no bad sectors, this is the state of art software for filesystem repair: ( http://www.alsoft.com/ )
You might consider to make a copy to another disk even when you use diskwarrior for safety reasons, but I never saw that diskwarrior made the situation worse. The program will stop if the disk cannot be repaired safely and, if you are lucky even might mount the damaged filesystem so you can copy your data.

Using gzip to compress the image is possible. However you then cannot manipulate or repair the gzipped image unless you copy it back to a disk. Also, it takes additional time to compress the data.
Disks are dirt cheap.
And like Nils suggested, you might need a spare disk anyway if you want to have the option to redo a repair or try other methods.

If you copy an rdisk device, all information including paritions will be copied.
If you make a copy to a larger disk with dd, the larger disk will be truncated to the size of the original disk. There is no easy way to get access to the additional capacity without reinitializing the disk.
However, you dont need to make a copy to a physical disk:
sudo dd if=/dev/rdisk2 of=/mydirectory/myimage.dmg bs=512 conv=noerror,sync
will make a perfect disk image, which you can mount, repair and check with apples disk utility to diskwarrior.

Is it possible, there is a FAT filesystem on this disk ?
What does the apple disk-utility tells you about the disk.

Or, what does "sudo pdisk --list" tell about the partitioning of the disk.

I dont think, the FW bridge causes the trouble. If you were able to dump the disk to /dev/null, most likely only the filesystem is in trouble.

Making a backup of the disk prior to any repair attempts is a good idea!

about the 128GB limit, from http://www.speedtools.com/ATA6.shtml, they have a driver which overcomes this:

"The Intech ATA Hi-Cap Support Driver software was created to allow the use of extended capacity ATA drives (drives greater than 128 Gigabytes in size) on older (Pre-Mirrored Door) G4 and G3 Macintoshes running MacOS X versions 10.2 and later. Without this software installed, any extended capacity drive which is connected to the native ATA bus on older Macintosh models will be limited to only 128 Gigabytes. Later model G4's and all G5's do not suffer from this limitation."

Just in case, you want to give some other software a try:
"Data Rescue" from www.prosofteng.com
This is not a repair, but a file recovery software for mac OS.
Even if due to whatever fatal error your partition table has been zeroed, this utility should find files to recover.
They have a limited trial version AFAIK.

Maybe, it tries to find something one the CD/DVD drive but fails since pdisk cant talk to the drive or there is no media in it ?

If you get bored by the long time the copy process is taking, you might start from the beginning and use a larger block size, like 10MB - but it should be ( i am not shure what happens if not) a multiple of 512 bytes. I did this too and the resulting copy was fine.

Thanks !

Good, that you finally could recover your data.
Phil