os-prober fails to output in chroot environment thus failing update-grub

[undisclosed lastname]

while in chroot, os-prober returns nothing, therefore update-grub fails to create entries for auxiliary OS. This happens on both copies (except fstab) of the same system on different disks, while it is working fine without a chroot (in both cases).

tested with and without mounting disk into chroot

Description: Ubuntu 12.04.1 LTS
Release: 12.04

os-prober:
  Installiert: 1.51ubuntu3
  Kandidat: 1.51ubuntu3
  Versionstabelle:
 *** 1.51ubuntu3 0
        500 http://de.archive.ubuntu.com/ubuntu/ precise/main amd64 Packages
        100 /var/lib/dpkg/status

I WAS on IRC and os-prober is not supported from grub team.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: os-prober 1.51ubuntu3
ProcVersionSignature: Ubuntu 3.2.0-36.57-generic 3.2.35
Uname: Linux 3.2.0-36-generic x86_64
NonfreeKernelModules: fglrx zfs zcommon znvpair zavl zunicode
ApportVersion: 2.0.1-0ubuntu17.1
Architecture: amd64
Date: Tue Jan 29 22:25:16 2013
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110427.1)
MarkForUpload: True
SourcePackage: os-prober
UpgradeStatus: Upgraded to precise on 2012-11-21 (69 days ago)