Unable to install Ubuntu 18.04 LTS using Multisystem LiveUSB

I am able to boot into Ubuntu 18.04 LTS from a Live USB, without issue, having created it from Multisystem. When I try and install it, having selected a partition to format and install to, it constantly complains that it can't unmount isodevice and so can't continue.

If I format the partition in Gparted and try and continue the installer, it pops up with that same dialog once and then continues. I fill in my information and on the next page I can see that it's not actually installing and stuck on "Detecting file systems".

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.4
ProcVersionSignature: Ubuntu 4.15.0-13.14-generic 4.15.10
Uname: Linux 4.15.0-13-generic x86_64
ApportVersion: 2.20.9-0ubuntu2
Architecture: amd64
CasperVersion: 1.392
Date: Sat Apr 7 04:31:52 2018
ExecutablePath: /usr/lib/ubiquity/bin/ubiquity
InstallCmdLine: BOOT_IMAGE=(loop)/casper/vmlinuz.efi root=UUID=3D0E-5547 maybe-ubiquity debian-installer/language=en keyboard-configuration/layoutcode=gb iso-scan/filename=/ubuntu-18.04-beta2-desktop-amd64.iso boot=casper file=/cdrom/preseed/ubuntu.seed noprompt quiet splash --
InterpreterPath: /usr/bin/python3.6
LiveMediaBuild: Ubuntu 18.04 LTS "Bionic Beaver" - Beta amd64 (20180404)
ProcEnviron:
 TERM=unknown
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
Python3Details: /usr/bin/python3.6, Python 3.6.5rc1, python3-minimal, 3.6.4-1
PythonDetails: /usr/bin/python2.7, Python 2.7.14+, python-minimal, 2.7.14-4
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)