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linuximagehard-drivedd

dd imaging a failing disk which drops connection

I am trying to perform an image on a hard disk which is failing. The issue I am encountering causes the program to fail as the disk will routinely drop during the image process and when it is re-recognised by the system it is under a different address (/dev/sdb is now /dev/sde).

I have tried imaging each partition independently but on a 500GB disk I am strugging to get past 100GB a session before the disk will drop (i think the head is going as it clicks).

My question is, if using dd is there a way to image the disk, breaking it down into say 50GB parts so that I can get the whole disk over a number of images and then consolodate.

Or better still, is there a way to force the disk to re-identify on the previous location?

I have found little information on this topic so any insight would be useful.

Thanks.

Solution

  • When the device is lost, your stream will be lost, too. You cannot recover it even if it gets the same device name assigned. However you might want to employ udev rules to get the same name back just for your convenience.

    In dd, you can use four useful parameters:

    • bs=BYTES the size of a "block"
    • skip=N number of blocks to skip in input
    • seek=N number of blocks to skip in output
    • count=N number of blocks to be copied (we don't need it here)

    Also, dd has the, albeit a bit hidden, feature of providing progress reports. You can either use "status=progress" or send a signal to the process. The latter is more complicated but it allows you to define the frequency of progress reports. For example, you can do this in another terminal:

    for ((;;)); do sleep 1; kill -USR1 `pidof -s dd`; done

    Putting all of this together, you can use bs=4M as a reasonable blocksize. Then you can run aforementioned command in a secondary terminal, then start dd, initially with

    dd bs=4M seek=0 skip=0 if=/dev/… of=…

    After it fails the first time, you use the last block number that was successfully copied by dd as parameters to seek and skip. You can be a bit conservative here (decrease the number a bit) to ensure you don't get any "holes" in your output.

    Repeat until the whole disk is done. Good luck!