grub-install failed while installing from usb

created an usb with disk creator; rebooted with usb (windows 8 already installed); chose option to install immediately; created a boot-partition, a swap-partition and a root-partition. Install started, but just before finishing raised the error grub-install /dev/sda8 failed

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: ubiquity 2.15.26
ProcVersionSignature: Ubuntu 3.11.0-12.19-generic 3.11.3
Uname: Linux 3.11.0-12-generic i686
ApportVersion: 2.12.5-0ubuntu2
Architecture: i386
CasperVersion: 1.336ubuntu1
Date: Mon Feb 3 21:56:39 2014
InstallCmdLine: noprompt cdrom-detect/try-usb=true persistent file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd.lz quiet splash -- maybe-ubiquity
LiveMediaBuild: Ubuntu 13.10 "Saucy Salamander" - Release i386 (20131016.1)
MarkForUpload: True
ProcEnviron:
 TERM=linux
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)