I accidentally used dd on my secondary hard drive while trying to write an install disk. The command was dd bs=4M if=foo.iso of=/dev/sdb

Fortunately, I umounted the flash drive and not the hard drive, so the drive was still mounted.

Following another forum post, I was able to pull

/dev/blocks/sdb/sdb1/start: 1197565952

/dev/blocks/sdb/sdb1/size: 4662966272

/dev/blocks/sdb/sdb2/start: 2048

/dev/blocks/sdb/sdb2/size: 1197563377

sdb1 was ext4, sdb2 was ntfs. I'm fairly sure it was gpt, but I don't know how to check.

That's as far as I got though. I'm a bit of a newbie, please give me step by step if possible.

  • How large is the ISO file you copied to the disk? Did you interrupt the process? If you did, how much did it copy?
    – Daniel B
    Commented May 22, 2018 at 8:53
  • @daniel It was a xubuntu 18.04 iso. Its about 1.3 gb. I allowed it to finish writing before I realized what i did. The previous partition table got overwritten
    – BabylFish
    Commented May 22, 2018 at 8:59
  • So I assume this is a 6 TB drive, correct? Are you positive the order of partitions in the table was switched from the physical order?
    – Daniel B
    Commented May 22, 2018 at 9:11
  • @daniel It is a 2.7 tb drive. I'm not positive about the order of the partitions. It was originally formatted on a windows machine. I used gparted to shrink the ntfs partition and move it all the way to the left. Then i made an ext4 partition in the space to the right.
    – BabylFish
    Commented May 22, 2018 at 9:16
  • Oh, the sizes are in 512-byte blocks, oops. Well, sounds like GParted didn’t fix the order. Was there any data of relevance on the NTFS partition (sdb2)?
    – Daniel B
    Commented May 22, 2018 at 9:24

1 Answer 1


Update: It turns out the drive was gpt and the backup gpt header and partition table were not corrupt. I was able to use gdisk to load and write the backup gpt header and partition table back onto the disk using this tutorial:


The ext4 partition was accessible via a manual mount from the terminal, but the ntfs partition was damaged. I had to use "chkdsk /f" from a windows install disk to fix the ntfs partition.

Hope this helps someone else in a similar situation

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