I use Acronis TrueImage.
Bootable: It can be installed on Windows, but also comes with a bootable DVD (shipped as ISO, additional cost if you want it physical), so you can create a copy without modifying your system. I don't know if they provide the ability to boot from USB stick in case you don't have an optical drive.
Commercial: It's neither free nor open-source. A license is at ~40 €.
1:1 clone: sector-by-sector cloning is possible. For partition types known to TrueImage, you could also skip the free space. In that case, you'll not be able to restore deleted files.
Larger target disk: TrueImage will copy the contents into a single file, so it does not care about the size of the disk. It can even compress the file so you save additional disk space and can keep several backup copies. As an alternative, you can also copy the partitions directly, destroying the file system on the destination disk.
In your case, the following approach may be best:
- Create a copy from the internal disk to the external, copying partitions directly (not as a file)
- Switch internal and external drives
- Boot from the cloned drive to see if everything is still fine
- If so, install the other dual boot OS
Like that you'll always have the original drive as a backup and you don't need to worry whether the backup is really functional.