In contrast, dd_rescue first attempts to identify the unreadable sectors. This is particularly useful for large disks because reading can take several hours. The program remembers its actions, so you can interrupt the read operation and then continue some time later. After it has copied the good blocks, it checks the bad blocks again and tries to read as much data as possible from them. The two programs differ in terms of their parameters and their approach: The newer ddrescue reads from the disk in quite large steps, skipping bad blocks first. GNU ddrescue has replaced the dd_rescue tool in many distributions in some repositories, you will even find both tools. You will generally need to install ddrescue via the package manager in Ubuntu, it is in the gddrescue package. The duplicate itself is created with a small tool named GNU ddrescue, which reads a medium bit-by-bit and stores the content into a file known as an image. If you need to save an entire hard disk instead of just a partition, you might need to install an additional empty, larger hard drive to recover the data. For a 2GB memory card, you should therefore have at least 6GB of free space on the rescue disk. Consequently, at least two, and preferably three, copies of the entire data storage medium should fit on the disk. Of course, the hard disk on the rescue PC must have enough free disk space for the copy, and you need to store the restored data somewhere. Therefore, it is advisable to first make a bit-perfect copy of the volume. The risk that the tools destroy even more of the data is simply too great, and then you have no way to make a second rescue attempt. Trying to restore files directly on the failed disk is not a good idea. The size of the partitions provides further evidence, as does the output by theįigure 3: The last entry in fdisk confirms that sde is the SD card. You can use the label column for orientation. The command outputs a list of all the connected media and their device files (Figure 1). Which you need to run as root, like all the other commands in this article. The command to help you with this is blkid -o list Next, you need to find out which device file you can use to access the defective medium. Do not mount the data storage medium, even if Linux suggests doing so. An SD card, for example, can be slotted into a USB card reader and plugged into the Linux PC. When Linux is running, connect the defective medium. If it does not have Linux installed, you can simply boot from any Live CD. Now you can boot the PC on which you want to perform the rescue. This method is quite common with swap partitions – or partitions that Linux incorrectly assumes to be swap. A hard drive or SSD with less important data can be removed and plugged into an external USB enclosure, which at least prevents the Linux system used to rescue data from arbitrarily using the same disk. For critical data, you can buy a write-protection adapter, as used in computer forensics. USB sticks, hard drives, and SSDs usually do not have a write-protect switch. The memory card from the digital camera, in the case initially described, turned out to be a slightly older 2GB SD card write-protecting it was easily accomplished by moving the small write-protect tab on the right side. This means that the rescue attempt cannot destroy the operating system, and the rescue tools cannot destroy the data. First, you should enable physical write protection for the defective medium as soon as possible.
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