Move my current Windows 10 system to a USB disk ?

Make sure to check the BIOS/UEFI settings for the order in which the device boots (the drive should be first)

For backup you can use the built-in backup and restore tool or third party tools. But restoring the image would require it to be reinstalled on a given computer. A computer needs computational power, CPU etc. so running an operating system only from a plugged in storage device wouldn't suffice. You need to tell the computational parts of the device to use that drive as the OS (i.e. reinstalling the OS). And if you have multiple options, you would need to specify into which OS the device should boot into. Also, the disk you're using for creating the backup needs to be formatted correctly. 

What are the steps you're taking to reinstall/restore the image? What is the purpose, do you need it as an additional backup in case of data loss or are you moving to a different PC? Maybe going thorugh the hassle and reinstalling apps isn't always a bad idea, if for example they are outdated and have vulnerabilities.

If you have created the backup image with the built in feature to create a system image backup, you can install Windows 10 (preferably on a different PC before erasing your existing one without knowing if the restoring process works) and choose the built-in restore options.