Using dd – disk cloning

dd (disk duplication) is a utility that can read raw data of a disk, even if the Mac doesn’t understand the filesystem.

I have used it before in p2v a physical to a virtual server. For details take a look at this article.

  • if= specifies input path (file, or device)
  • of= specifies output path (file, or device)
  • bs=n sets both input and output block size (optional, default=512 byte blocks)
  • conv=noerror,sync tells dd to be fault-tolerant and ignore read errors (optional)

If the operation stops with an I/O error, trying to salvage all readable data with conv=noerror,sync.
This option can often recover a dead hard drive or an unreadable file, but it does not repair the error.

Make a clone of a disk:

In this case, I was fooling around with a Microsoft StorSimple appliance and wanted to have a backup of it to go back to if I messed it up too badly. This is not the first time I have done it, and sure not the last time, and always forgot the commands, so to future Jesper, here goes.

Open Terminal and try this:

  1. Attach and identify the source disk:

2. if mounted, unmount the source disk:

Copy the source disk to the desktop:

If you want to save space, make it gzip it with:

Failed disk backup:

If the copy fails due to disk errors, try using the following command at step 3:

dd will take much longer using this option, errors are written as null bytes. fsck / chkdsk the disk afterward.

Notes:

  • Using /dev/rdisk# (raw disk) in place of /dev/disk# is much faster
  • You can check progress, while the command is running, by pressing Ctrl-T
  • If you can attach both disks, copy directly: sudo dd if=/dev/disk2 of=/dev/disk3 bs=64k

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Jesper Ramsgaard