how to edit a disk image to make it mountable
 

This is partly hardware and partly software, so this seems like the right forum.

The underlying problem: my USB hard drive containing multimedia (mp3's, ripped movies, etc) crashed. My backup was two years old (oops).

Using dd fails. Using ddrescue shows that the first eight blocks are bad, but the rest seems to be extracting well. This is part of a VFAT partition (so I could mount it under multiple operating systems).

How can I rebuild the first eight blocks so that mount -t vfat will work?

Or am I being obtuse, and that there is a better way?

--woody

testdisk has a good reputation as a disk rescue program: http://www.cgsecurity.org/wiki/Damaged_Hard_Disk

First make an image of the failed drive if you can. I would recommend Clonezilla/ for creating the image.

Then I would try SpinRite (Level 4)

I have used it to successfully restore at least 20 drives back to a working state. A few of them are still working years later and I now use Spinrite before I put a new drive into service to make sure it will reliably store data.

I do have to say SpinRite has outright killed a couple of drives that had mechanical problems, which is why you should get a drive image first if you can. It even killed a new drive, which was replaced under warranty, before I had even put the drive into service.

Disclaimer: I am not affiliated with Spinrite in any way, just a satisfied user.

Good Luck!

Thanks, mostlyharmless. Am trying testdisk now. Didn't find things first time through, but its trying again. "Read error at 0/1/7 (lba=69)" is not encouraging.

Never, I did image the bad partition, using ddrescue. I am now the proud owner of a 107397448704 file, "multimedia.img".
mount -t msdos -o loop multimed.img /mnt
fails, without a useful error message.
fsck.vfat multimedia.img
fails, with "Logical sector size is zero."

What I was thinking is that I could image a partition that does have some files, and sew the first eight blocks onto my image, and see if that helps?

Might work, but I don't know enough about the file fragmentation and what is on those 8 failed blocks to know. Can't hurt as long as you keep the original image to try something else if it doesn't work.