I was recently messing around with the dd program to write ISO images to USB drives, instead of wasting DVDs or CDs. The command to do this is as follows:

# dd if=archlinux-2010.05-netinstall-x86_64.iso of=/dev/sdX

Where sdX is the device name of the disk you wish to write the image to.

This is alright if you only have two disks in your computer, where you would almost certainly want to write to /dev/sdb. However, if you have more than two disks, you need to be absolutely sure that you have selected the correct device — choose the wrong disk, and its data will be overwritten by that of the ISO image.

This is precisely what happened to me (twice, in fact!). I thought I was writing to my flash drive but it turned out that I was actually writing to my 1TB external backup drive. Luckily I had the presence of mind to kill the dd process after I realized that the activity light on the flash drive was not blinking, but the 80MB or so of data that was written was enough to destroy the MBR. That’s a big problem because the MBR acts like a map telling the computer where data is located on the drive. Without that map, the drive is useless.

Thankfully, one Christophe Grenier has written a program called TestDisk which scans the raw data of the hard drive and searches for lost partitions. I was able to recover the lost partitions on my drive using this fantastic tool.

It uses a few different methods to recover the lost partitions. First, it quickly scans through the entire hard drive, looking for lost partitions. Then, it gives you the option to perform a deeper search, which inspects the drive more carefully. Finally, it checks if there is a backup MBR on the device, and if so, uses it to replace the lost MBR.

This final option worked for me. Personally, I think this should be the first step performed, as it takes several hours to scan the entire drive twice whereas it presumably takes less than a second to inspect the backup MBR.

Check out this step-by-step guide detailing its use.