Hack and / - When Disaster Strikes: Hard Drive Crashes
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.
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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