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Very similar question to this one here

I have a Mid-2015 MacBook Pro - A1398 (EMC2910). Without going into details why, I would like to save an exact, sector by sector copy of the entire drive (currently running El Capitan and Windows 10 via Boot Camp) to another location as a single image file, without logging-in at all, and restore it back as-it-was at a later date, as if the system has never been touched. After imaging the SSD, I want to erase the entire drive and reinstall and update macOS to the latest version (Big Sur). That means a Boot Rom update, too, and it worries me a bit when it comes to restoring the system back from the image. Does this sound doable? I can get a 12+16 to PCI-E adapter and clone the drive that way using another Linux/PC system, but if it's possible without disassembly (using an external boot drive + another one for storing the created image) that would be great.

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You can definitely clone the drive. The easiest way is probably this:

  1. Boot the Mac in Internet Recovery mode (hold Option-Command-R during boot)

  2. Unmount the main drive if it is automatically mounted

  3. Plug in an external USB drive for storing the image

  4. Open Terminal and copy the drive using a command something like this:

dd if=/dev/rdiskX of=/Volumes/MyUSBDisk/image.img bs=512k

where X should be replaced with the actual disk number of your main drive (check and double check with Disk Utility in advance) - and MyUSBDisk should be replaced with the name where your USB drive is actually mounted.

This will give you an image file with the contents of the drive, which you should be able to roll back without any problems.

However, I don't think you'll be able to easily roll back the Boot ROM update. It is usually not a practical problem though, as upgrading usually still allows booting the old software.

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