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I need to clone a disk. usually i've been doing this straightforward with dd. But in this case the disk i need to clone is an external USB-Disk that is 5TB large. So I'm guessing this will take somewhat between "way to long" and "an eternity".

Is there a cloning method that supports resuming (if something goes wrong, the PC crashes or I need to stop the process for some reason)?

The disk in question contains only one ext4 partition and is basically a data dump, so nothing complicated there.

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    ddrescue does it. Just shooting from the hip. – Appleoddity Sep 25 '18 at 16:33
  • Do you need a complete disk image? What about file-based programs like rsync? Rsync doesn't support sessions / resuming, but can quickly skip over files previously sync'd. – Stanley Yu Sep 25 '18 at 16:34
  • ddrescue, yes; but not if "some reason" involves writing to the device being cloned. – Kamil Maciorowski Sep 25 '18 at 16:40
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You can use rsync on the underlying file system, but you can also use it on the disk partition itself. From this discussion on Unix stackexchange it looks like the --partial-dir option could help you.

There are also options in dd that could help: seek=N and skip=N, while also using append mode.

  • Not sure I'd trust any disk image which was made in multiple steps. On my home PC, using Macrium Reflect, imaging 3 TB takes about 6 hours with verification turned on, so depending on system and HDD speed, 5TB should be done in less than 12 hours. – DrMoishe Pippik Sep 25 '18 at 22:37

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