unable to install grub to /dev/sda

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I get an"unable to install grub to /dev/sda error when trying to install ubuntu 15.04 with windows 10 already installed.

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 x86_64
NonfreeKernelModules: wl
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CasperVersion: 1.360
Date: Sat Aug 15 17:54:47 2015
InstallCmdLine: bootkbd=us console-setup/layoutcode=en_US console-setup/variantcode=nodeadkeys locale=us_us persistent noprompt cdrom-detect/try-usb=true file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd.lz splash -- maybe-ubiquity
LiveMediaBuild: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
ProcEnviron:
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)