How do I clone an installation of Windows 10 from macOS?

Solution 1:

You can use the dd command to clone the disk as follows:

If the source SDD is larger than the destination SSD, start up your Mac in Windows 10 and shrink the partition on the source disk, as explained in this Microsoft document: Shrink a basic volume, so that its size is smaller than the destination SSD. Then restart your Mac in macOS.
Connect both SSD disks to your Mac

If you get a message similar to this when connecting the destination disk:

select "Ignore".
Launch Disk Utility (located in the Applications > Utilities folder).
Select the source disk and write down the device name.

In the screenshot below, the device name is disk2:
Select the destination disk and write down the device name:

In the screenshot below, the device name is disk3:
Select the source volume and unmount it.
Select the destination volume and unmount it. (This step will not be necessary if were prompted to select "Ignore" in step 1.)
Launch Terminal (located in the Applications > Utilities folder).
Run:

sudo dd if=/dev/diskX of=/dev/diskY bs=1m

where:

diskX is the device name of the source disk (in the example above, disk2)

diskY is the device name of the destination disk (in the example above, disk3)

sudo is a command that let's you run the dd with administrative privileges.

Type your password if requested.

It is extremely important that you don't mix up the device names of the disks! Otherwise you will overwrite your Windows 10 disk.

This may take a while, so be patient. You can check the progress of dd as explained in this answer: How can I track progress of dd
Test the destination SSD. You may want to extend the volume as explained in this Microsoft document: Extend a basic volume