This is the sources.list on target when I install without configuring the network at the moment of grub fail.
If I uncomment the squeeze line and retry, grub-pc is found and installation is OK. ==================<cut>===================== # # deb cdrom:[Debian GNU/Linux 6.0.0 _Squeeze_ - Official Snapshot i386 LIVE/NETINST Binary 20110103-10:39]/ frozen main # deb cdrom:[Debian GNU/Linux 6.0.0 _Squeeze_ - Official Snapshot i386 LIVE/NETINST Binary 20110103-10:39]/ squeeze main # deb cdrom:[Debian GNU/Linux 6.0.0 _Squeeze_ - Official Snapshot i386 LIVE/NETINST Binary 20110103-10:39]/ stable main # deb cdrom:[Debian GNU/Linux 6.0.0 _Squeeze_ - Official Snapshot i386 LIVE/NETINST Binary 20110103-10:39]/ testing main # deb cdrom:[Debian GNU/Linux 6.0.0 _Squeeze_ - Official Snapshot i386 LIVE/NETINST Binary 20110103-10:39]/ unstable main # Line commented out by installer because it failed to verify: #deb http://security.debian.org/ squeeze/updates main # Line commented out by installer because it failed to verify: #deb-src http://security.debian.org/ squeeze/updates main ==================<cut>===================== So, it seems that the problem is when sources.list is generated.