Warning
dd is a powerful but also dangerous tool. Check and double-check, that everything is correct before you launch the command lines! dd does what you tell it to do without questions. A small mistake (typing error) can make you overwrite and destroy valuable documents, for example the family pictures.
Command lines
Text after the # character is a comment for the human eye, not used by the shell interpreter.
If you are sure that /dev/sdx is the correct target device, you can use the command
sudo dd if=/dev/sda of=/dev/sdx bs=64K # should be upper case K
to clone the drive to the device /dev/sdx, where x can be b, c, ...
If you want to create an image file (without compression), you can use the command
sudo dd if=/dev/sda of=dd-clone.img bs=64K # should be upper case K
If you want to create a compressed image file, you can use the command
sudo -s # to get the root prompt `#`
dd if=/dev/sda bs=64K | xz -c > dd-clone.img.xz
exit # to get the user prompt `$`
Such a compressed image can be extracted with
sudo -s # get the root prompt `#`
xzcat dd-clone.img.xz > dd-clone.img # get a big uncompressed file
xzcat dd-clone.img.xz | dd of=/dev/sdx bs=64K # clone to `/dev/sdx`
exit # get the user prompt `$`
Tips
If you want the compression to be more efficient, you should overwrite the free drive space with zeros. You can do that in the following way:
Use zerofree for linux ext partitions.
Mount other partitions and use the following command lines for partitions with other file systems. Let us assume that you have mounted a partition at the mountpoint /mnt, and that the whole drive is used for partitions. Check that you have mounted a partition there! Otherwise you will fill the root partition and your running operating system will stop working.
sudo dd if=/dev/zero of=/mnt/blank bs=4096 # Let it fill the partition
sudo rm /mnt/blank
See Tools at this link: SanDisk SSD Plus: Half the performance on Linux than on Windows?
Alternative
Clonezilla is an alternative to dd. It is safer and it is faster, particularly if there is a lot of free space. Clonezilla will only copy used blocks (and skip free blocks), which will make the copy much faster, particularly if a fair part of the main partition /dev/sda3 is free (not used by files).
dd(wrapped by mkusb to avoid mistakes) or a directory with several files as by Clonezilla), or 2) a clone (an exact copy of the internal hdd, so that the external drive can replace the internal one)? In both cases you can usedd(with or without wrapper) and Clonezilla. I would recommend Clonezilla, because it will only copy used blocks (and skip free blocks), which will make the copy much faster, particularly if a fair part of the main partition /dev/sda3 is free (not used by files).bsis the block size while cloning withdd. It does not affect the block size or sector size on the target drive, but it can affect the speed of the operation. I usebs=4096, which I find a good value in most cases. The default isbs=512, which makesddslow in current linux operating systems. But be warned:ddis a powerful but also very dangerous tool. It does what you tell it to do without questions. A small mistake (typing error) can make you overwrite and destroy valuable documents, for example the family pictures.