Hack and / - When Disaster Strikes: Hard Drive Crashes

All is not necessarily lost when your hard drive starts the click of death. Learn how to create a rescue image of a failing drive while it still has some life left in it.
Mount the Image

Once the fsck has completed, I can attempt to mount the filesystem and recover my important files. If you imaged to a complete hard drive and want to try to boot from it, after you fsck each partition, you would try to mount them individually and see whether you can read from them, and then swap the drive into your original computer and try to boot from it. In my example here, I just want to try to recover some important files from this image, so I would mount the image file loopback:

$ sudo mount -o loop /mnt/recovery/sda1_image.img /mnt/image

Now I can browse through /mnt/image and hope that my important files weren't among the corrupted blocks.

Method of Last Resort

Unfortunately in some cases, a hard drive has far too many errors for fsck to correct. In these situations, you might not even be able to mount the filesystem at all. If this happens, you aren't necessarily completely out of luck. Depending on what type of files you want to recover, you may be able to pull the information you need directly from the image. If, for instance, you have a critical term paper or other document you need to retrieve from the machine, simply run the strings command on the image and output to a second file:

$ sudo strings /mnt/recovery/sda1_image.img >
/mnt/recovery/sda1_strings.txt

The sda1_strings.txt file will contain all of the text from the image (which might turn out to be a lot of data) from man page entries to config files to output within program binaries. It's a lot of data to sift through, but if you know a keyword in your term paper, you can open up this text file in less, and then press the / key and type your keyword in to see whether it can be found. Alternatively, you can grep through the strings file for your keyword and the surrounding lines. For instance, if you were writing a term paper on dolphins, you could run:

$ sudo grep -C 1000 dolphin /mnt/recovery/sda1_strings.txt >
/mnt/recovery/dolphin_paper.txt

This would not only pull out any lines containing the word dolphin, it also would pull out the surrounding 1,000 lines. Then, you can just browse through the dolphin_paper.txt file and remove lines that aren't part of your paper. You might need to tweak the -C argument in grep so that it grabs even more lines.

In conclusion, when your hard drive starts to make funny noises and won't mount, it isn't necessarily the end of the world. Although ddrescue is no replacement for a good, tested backup, it still can save the day when disaster strikes your hard drive. Also note that ddrescue will work on just about any device, so you can use it to attempt recovery on those scratched CD-ROM discs too.

Kyle Rankin is a Senior Systems Administrator in the San Francisco Bay Area and the author of a number of books, including Knoppix Hacks and Ubuntu Hacks for O'Reilly Media. He is currently the president of the North Bay Linux Users' Group.

______________________

Kyle Rankin is a director of engineering operations in the San Francisco Bay Area, the author of a number of books including DevOps Troubleshooting and The Official Ubuntu Server Book, and is a columnist for Linux Journal.

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Partially unrecoverable allocation table

lopo's picture

Tried it all but my rescued image does not show any file under /home/lopo although they are there. On an 92GB partition on less than 700KB are damaged but it was enough to lost everything ;(

Foremost does not reconigzed a lot of my files: bz2, gz, svg, odf files, etc., so -t all is not really ALL.

dd progress report with USR1 signal

RALi's picture

While I have no quibble with using ddrescue instead of dd (the wonderful thing about *nix is that there are 72 different ways to do anything) I do have to comment on your statement: "In fact, in some circumstances, I prefer using ddrescue over dd for regular imaging as well, just for the progress output."

dd provides progress info if you send a USR1 to the process. From the dd(1) man page:

Sending a USR1 signal to a running `dd' process makes it
print I/O statistics to standard error and then resume copying.

I guess I'm lucky

goblin's picture

I guess I'm lucky then.
I've never had a hard drive fail on me. Ever. Not at work, not at home.
I still keep my first PC around, with its fully functional hard drive in it (a 80486 80MHz with a 528 MB hard drive, from 1994 I think).

A Better 'Method of Last Resort'

mmueller's picture

If you can't mount a ddrescue image, but need to retrieve documents, photographs, pdf files, ect. you can use a nifty program called "foremost." It is available for most *nix platforms, and on Windoze via cygwin. Foremost scans through a hard drive image, mountable or not, and looks for recognizable file headers. It understands over twenty popular file headers including jpg, pdf, doc, xml, etc. When it finds these files it dumps them out as usable files. It is truly a thing of beauty. The first time you use it, you will just sit back in amazement. For example, if you have a hard drive image named my_hd_image.dd, that was made with one of the dd utilities, you could execute the following command:

~$ foremost -t all -i my_hd_image.dd

After the command executes, a subdirectory will be created that has all of the recovered files, organized neatly by file type. On Ubuntu, you can get foremost by typing:

~$ sudo apt-get install foremost

This tools is also excellent for recovering deleted files from USB drives, etc. Enjoy!

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