Source - https://old.reddit.com/r/computerforensics/comments/ul8e7r/a_starters_guide_on_recovering_damaged_and_rotten/
TL;DR: I'm Using ddrescue/dvdisaster/testdisk and photorec to recover data from a disc rotten CD
Prettier version of this post is available here.
The First Hurdle-Reading data from a Damaged CD / DVD
The first problem anyone’s with a damaged disc going to encounter, is that they cannot copy files from it using a regular copying mechanism (eg:. file explorer, terminal commands).
This is due to the fact that, normal file copying mechanisms will not attempt to read from a bad sector or unreadable data. Instead, they will freeze, or throw an error upon encountering such data.
To recover data from a damaged medium, we need specialized tools that are aware of this problem and will continue with the reading process, even after encountering errors.
Three of such tools are ddrescue , dvdisaster and testdisk.
These applications can be used to create an exact image file (or something that resembles it), from a damaged medium.
Ok. I have some good news and some bad news. Let’s get the bad news out of our way first.
Note: Reading from a damaged disc using ddrescue or dvdisaster is going to take a long time. For comparison, from my CD, which has the capacity of 700 MB, there was 400MB of data. And it took around 12 Hours for it to finish reading it!
But the good news is that, the process can be cancelled and resumed at any time!
The Plan
You can select either dvdisaster, ddrescue or TestDisk to begin the read process.
Once we get an image file, we will carve out the readable data using the photorec utility.
Let’s start the reading process.
I’m going to talk about creating a disk image using the three applications I’ve mentioned. Although the end goal is same, knowing multiple applications to do the same thing can come in handy.
Creating Disc Image using DVDisaster
Dvdisaster is a GUI application, so that would be easier to use. It has a nice interface and a cool animation to display progress.
On Debian based distros, we can install it using
sudo apt install dvdisaster -y
Using DVDisaster is easy. Just select the CD / DVD reader, Specify location to save the output file and Click on start Reading. Once the reading is finished, the file will be stored in the specified location.
Creating Disc Image using DDrescue
Sometimes, we would need to manually specify some settings when reading a damaged disc; like the block size, reading direction etc.
If you want such granular control while reading the disc, then you should go with ddrescue. Also the chances of getting succesful data recovery are higher with ddrescue, since we can use it to run multiple times on the same disc with different read options.
On Debian based distros, we can install it using
sudo apt install gddrescue -y
We are going to run ddrescue three times on the disc. First, we are going to to make ddrescue skip the parts with error and we will read the good data.
Then on the second run, we are going to make ddrescue read the entire disc, including the blocks with errors and try to get more data from the disc.
And on the final run, we are going to make ddrescue read the entire disc, but backwards.
This three step recovery process ensures that we are going to get every last bit of readable information from the damaged disc. Read this answer for a detailed explanation.
Note: You can use the info ddrescuecommand anytime to get a great guide on how to use ddrescue.
Read #1 Reading just the Good Data
ddrescue -b 2048 -n -v /dev/cdrom dvd.iso rescue.log
This command specifies the following things
b : Block Size as 2048 (default blocksize of DVD)
n : No scrape ( Skip the bad sectors)
v : Verbose
/dev/cdrom : The path to mounted CD (This will vary on distros)
dvd.iso : Output image to write
rescue.log : Log file
Note: Now this reading process is going to take a looooong time. Either keep your computer running untill it finishes or just cancel and restart the process with the same command later, to resume the reading process.
Read #2 Reading the Bad Data
Once it has been finished, we can run it again with scraping (Reading the bad blocks), with the following command. Please note that we are using the same image file (dvd.iso), we’ve created in the first run and not a different file.
ddrescue -b 2048 -d -r 3 -v /dev/cdrom dvd.iso rescue.log
Here, we have two new flags.
d : Read the device directly (Instead of going through kernel)
r : Retry count on error
Now this is going to attempt to read and recover data from bad sectors. Once new data is found, it will be appended to the dvd.iso image we’ve created in the first step.
Let’s continue the waiting game..
Read #3 Reading the Bad Data, But in Reverse
Once that command has finished it’s execution, let’s scrape again, but this time in a reverse order.
ddrescue -b 2048 -d -R -r 3 -v /dev/cdrom dvd.iso rescue.log
* R : Reverse read
After this command finishes execution, we will have an image file with the data recovered by ddrescue.
Edit: Reverse read is not required, as ddrescue automatically does this.
Thanks LinAGKar
Now if we check the file type of the extracted image, we can see that it’s not a proper ISO file. Instead, it’s recognized as a data file.
This means that we have to perform additional carving in the recovered image, to get usable files from it.
Creating Disc Image using TestDisk
Now as the final method for creating a disc image file, I’m going to use Testdisk.
TestDisk is a free and open source tool, that helps users recover lost partitions or repair corrupted filesystems. Testdisk is actually faster in creating disc images, in comaprison to other disc imaging methods.
PhotoRec is a free and open-source tool for data recovery using data carving techniques, designed to recover lost files.
We will first use TestDisk to create a disc image from the CD. Then, we could use PhotoRec on the disc image to carve files from it.
To install TestDisk and Photorec on debian based distros, use
sudo apt install testdisk -y
Photorec is a part of testdisk suite. So, It will be automatically installed along with testdisk.
To use testdisk, simply pass the full path to my CD, as an argument to it. In my case, it is /dev/cdrom.
testdisk /dev/cdrom
Select Proceed and press enter when TestDisk prompts to select Media.
Now, choose Continue, when prompted to continue.
Select None when TestDisk asks for partition table type.
Now press Right Arrow to highlight the Image Creation at the bottom and Press Enter.
Select the directory to save the output disk image. If you want to save it to the current working directory, just press “C” to confirm.
Once TestDisk finishes creating the image file, we can choose to perform additional operations on it, or just exit.
Here, I am quitting TestDisk.
Now, we will have an image file named image.dd.
Now, let’s start the carving process.
Carving files using Photorec
To start the file carving process, we are going to use the tool photorec. If you’ve installed testdisk, photorec will be automatically installed along with it.
We can now run photorec on any of the disc images we’ve generated earlier to start the carving process.
photorec image.dd
Photorec’s interface is similar to TestDisk's.
Select Proceed and press Enter.
You can now select Search to start the File recovery.
OR you could choose specific file formats to recover from the File Opt menu.
After selecting Search, photorec will prompt you to choose the file system type. Choose Other and press enter.
Now, PhotoRec will ask us to select a location to save the recovered files. Select the location and press “C” to confirm.
Once the process has been finished, we can find the files inside a directory called recup_dir.*
Result
Around 30-40% of the files are recovered fully, others were recovered partially and some files have been split into multiple files.
Though it might not seem like a great number, considering the stage the disc was in, it is indeed a great achievement achieved through pure software trickery!
I’m sure that I could’ve gotten more data, if I’ve spent some time on physically polishing the CD’s surface to reduce the scratches. But, since this was just a hobby project, I’m more than satisfied with the outcome.