Recovery Needed

Krrr, krrr …! At least things are clear-cut when a hard disk gives up the ghost: You toss the offending disk, get a new one, and put the backup on it. However, those undead data media – that trick people into continuing working on them with no idea of the potential impact – are a real pain.

I recently determined that an SDHC card in my camera saves one out of 20 images (on average) as a colorful mess of pixels. I do know that memory cards give up the ghost sooner or later. However, I didn't realize that my camera could save to two cards simultaneously – a feature I stupidly didn't use. But, I'm all the wiser now.

What if really important data is stored on a haywire device that you just can't get rid of? This is where ddrescue [1] comes in. The tool is already quite ancient, but its developers look after it untiringly and adapt it to new types of data media. (It should not be confused with the even older dd_rescue.) Ddrescue is officially named GNU ddrescue; the packages on Debian and derivatives are therefore dubbed gddrescue.

The tool is included with many popular distributions. The first two letters subtly indicate a relationship with dd, and ddrescue actually generates a data medium's or partition's image. Unlike dd, however, it can't be stopped by read errors; instead, it stubbornly saves everything that it can get its teeth into.

Two-Speed Transmission

Administrators usually use ddrescue in two phases. The first phase involves creating an image with all the data that can be accurately read. In the following example, /dev/sdd1 is a partition with read errors on a USB flash drive:

sudo ddrescue -n /dev/sdd1 /home/charly/stick.img logfile.log

The second, more time-consuming phase involves using the tool to sort through the faulty blocks and save as much data from them as possible. The command is just the same as before, except you leave out the -n parameter. In the wake of ddrescue, there is still an armada of other parameters that control the tool's behavior.

There is also a GUI [2] that you can use to make some quick, useful default settings. I installed it quickly on my test Ubuntu using these three steps:

sudo add-apt-repository ppa:hamishmb/myppa
sudo apt-get update
sudo apt-get install -fym ddrescue-gui

As Figure 1 shows, the interface is businesslike and functional. The GUI sets the important parameters, but not all of them by far. Although I hope no one will need to use ddrescue permanently, the GUI is nevertheless a real help.

Figure 1: The genuinely helpful front end for ddrescue is DDRescue-GUI, which graphically implements the important parameters.