how to backup iphone to external usb drive

You cannot directly backup an iPhone to an external USB drive.

What you can do is backup your iPhone on a computer and then transfer the backup file to a USB drive. After that, you still cannot restore your iPhone from a backup on a USB drive. You would then have put the file back on your computer to restore from.

In the end, backing up on a USB Drive is not actually worth it. I recommend that you backup on a computer and/or backup to iCloud.

Axel F.