xubuntu 15.04 installation crashes writing grub

[kairuri]

create install USB stick with unetbootin
install xubuntu fails from boot menu
wait for desktop
install xubuntu 15.04 button fails writing grub to /dev/sda

sda1 is windoze
sda2 is swap
sda3 is btrfs

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: ubiquity 2.21.25
ProcVersionSignature: Ubuntu 3.19.0-15.15-generic 3.19.3
Uname: Linux 3.19.0-15-generic i686
ApportVersion: 2.17.2-0ubuntu1
Architecture: i386
CasperVersion: 1.360
Date: Fri May 1 15:30:00 2015
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz initrd=/casper/initrd.lz file=/cdrom/preseed/xubuntu.seed boot=casper only-ubiquity quiet splash ---
LiveMediaBuild: Xubuntu 15.04 "Vivid Vervet" - Release i386 (20150422.1)
ProcEnviron:
 LANGUAGE=en_NZ.UTF-8
 PATH=(custom, no user)
 LANG=en_NZ.UTF-8
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)