GRUB2 installation fails (4k sector ext. HDD)

The installation of Precise Pangolin fails at the last step - GRUB refuses to install itself in the MBR of the external HDD. In Oneiric Ocelot I tried to work that around by selecting a 512-byte-per-sector USB flash disk instead to install GRUB to, but in vain, the installation failed again. The one and only effective workaround was to install the entire system to that USB flash disk. Interestingly enough, the rest of the "GRUBless" system on the ext. HDD was fully accessible from there...

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: ubiquity 2.9.29
ProcVersionSignature: Ubuntu 3.2.0-19.30-generic-pae 3.2.11
Uname: Linux 3.2.0-19-generic-pae i686
ApportVersion: 1.94.1-0ubuntu2
Architecture: i386
CasperVersion: 1.311
Date: Sun Mar 18 14:14:15 2012
InstallCmdLine: initrd=/casper/initrd.lz file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash -- BOOT_IMAGE=/casper/vmlinuz
LiveMediaBuild: Ubuntu 12.04 LTS "Precise Pangolin" - Alpha i386 (20120318)
ProcEnviron:
 LANGUAGE=de_DE.UTF-8
 TERM=unknown
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)