External HDD – Using the dd Command on External Hard Drive with Partitions

ddexternal-hdd

I want to write a .raw file to the first partition of my external drive. I tried:
dd if=file of=/dev/sdb3 but it didn't give me a bootable drive since the correct command should had been: dd if=file of=/dev/sdb.

My question is would this command destroy data on the other two partitions that I have on the drive, or only on the first partition?

Best Answer

It would destroy the partition table, since that's stored at the beginning of the drive.
It would destroy as much drive contents as the size of the image itself. If it is large enough to reach into the second or third partitions then it will destroy those.
MEMDISK will allow you to leave it as an image and boot it directly.

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Ubuntu – move system to new SSD with dd command

Any time you use dd (a.k.a. disk destroyer) there is a potential for messing up your data. dd has no safeguards for accidentally writing your new drive over the old drive. The only thing keeping it working right is if you're VERY careful with typing the commands correctly and not mixing up the source and destinations. if= and of= are only one small mistyped keystroke away from each other!

The other problem is that dd is slow and puts unnecessary wear on your target SSD. Most partitions are typically no where near 100% utilization. Using dd to copy a partition with lots of free space ends up copying EVERY BYTE of said partition, even the bytes that aren't actually containing any allocated data!

Here's what I would do if it were my system:

Take a screenshot of your old SSD's partition table for reference later when we set up your new SSD's partition table.
Connect the new SSD to the computer and disconnect the old SSD. Keeping the old SSD disconnected makes it impossible to destroy any of our precious data while we do potentially destructive work.
Boot from the Ubuntu installation USB thumb drive in Linux live mode. Make sure your BIOS always boots the thumbdrive in legacy/MBR mode instead of UEFI mode, since it sounds like your old system is using MBR. If my assumption is invalid, make sure your BIOS boots the thumbdrive in UEFI mode instead. Mixing things up would make it a lot harder to get grub to install properly later on.
Use gparted to set up your new SSD with the desired partition table and format the filesystems. Try to keep everything the same as your old SSD's partition table (aside from partition sizes and UUID's). Don't forget to mark the / root filesystem as active / bootable, just like your old SSD.
Now that all the dangerous partition editing stuff is done, you can safely shutdown and add the old SSD connected to the computer.
Boot the Linux live USB thumb drive again with both drives connected this time. Do not boot from the old SSD (we need to keep that system inactive to make a good clean copy of it).
Once booted, open a terminal and start working through these commands to copy the data from your old SSD to your new SSD. Unlike using dd, the cp command is far more error-proof because you can't destroy anything copying an empty file system over top of your old file system -- there is nothing copied in that case! Plus, we can mount the old SSD in "read-only" mode to avoid any possibility of modifying the old SSD.
```
mkdir oldroot newroot oldhome newhome
sudo mount -o ro /dev/sda1 oldroot
sudo mount -o ro /dev/sda2 oldhome
ls oldroot
ls oldhome
```
Verify that you can see your old data. If all you see is an empty file system, perhaps you've got the wrong "/dev/sdXX" device...
```
sudo mount -o noatime /dev/sdc1 newroot
sudo mount -o noatime /dev/sdc2 newhome
ls newroot
ls newhome
```
Verify that all you see is an empty file system as your destination. If everything is good, issue the following commands to copy all of your files while preserving all the permissions/ownership/SELinux context settings:
```
sudo cp -Rfax oldroot/* newroot/
sudo cp -Rfax oldhome/* newhome/
```
Once that's done, unmount everything and sync cached writes to persistent storage:
```
sudo umount oldroot
sudo umount oldhome
sudo umount newroot
sudo umount newhome
sync
```
Now you can safely shutdown and disconnect the old SSD so that we can't possibly destroy anything on it while we work through making the new SSD bootable.
Boot the computer using the USB thumb drive in Linux live mode with only the new SSD connected. Open a terminal and start working through these commands:
```
mkdir newroot
sudo mount -o noatime /dev/sda1 newroot
sudo chroot newroot /bin/bash
mount -t devtmpfs udev /dev
mount -t proc proc /proc
mount -t sysfs sysfs /sys
nano -w /etc/fstab
```
Now you need to modify the fstab file to match the UUID numbers to whatever they are on your new SSD. You can open gparted to look at your partition table and figure out what the new UUID's are. When done, hit CTRL-X and save the fstab changes to disk.
```
update-grub
grub-install /dev/sda
```
If everything is working right, this should configure grub and install it into your new SSD's MBR. After grub is installed, you have to unmount all the stuff we previously mounted from within the chroot environment before exiting. Otherwise, Ubuntu will have a hard time unmounting the Linux partition cleanly when you shutdown.
```
umount /sys
umount /proc
umount /dev
exit
sudo umount newroot
sync
```

And there you have it! Hopefully your new SSD is bootable and everything is good now.

Best Answer

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Ubuntu – move system to new SSD with dd command

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