I have a laptop with Windows 10 installed on the internal SSD. I wish to install Ubuntu on a 64 GB USB 3.0 external flash drive. I already have a separate bootable flash drive with ubuntu 17.04 on it. When I go through the installation process the partition manager detects all my drives and partitions but refuses to let me install onto the external USB drive without modifying the SSD windows boot partition. Several years ago I had a set up where booting a machine without an Ubuntu 14 USB would take me straight to windows and booting with the USB would take me into a persistent Ubuntu 14 that behaved just like a normal internal drive installation (no file or program or setting loss like live versions). I wish to replicate this. Thank you for your time.
TLDR; I have windows on an internal drive. I want ubuntu on a USB flash drive without touching the internal windows drive in any way. UEFI bios boot order is already configured.