I could install Mageia 1 beta 2 x86_64 on an external USB disk, but it does not boot

I downloaded the x86_64 DVD image and put in on an USB stick with the command "dd".
I could boot from this USB stick and install beta 2 on the external USB disk /dev/sdc.
/dev/sda is the notebook's internal harddisk
/dev/sdb is the USB stick
(Once you get during install in the partitioning screen, you can select the disk.)
I installed the bootloader on /dev/sdc1.
But I can not boot from the USB disk (now /dev/sdb as USB stick is no longer plugged in), I get
error: file not found
grub rescue>
By booting Mandriva 2010.2 and inspecting the content of /boot/grub on the external USB disk, I see that device.map still refers to sdc:
(hd0) /dev/sdc
(hd1) /dev/sda
(hd2) /dev/sdb
I changed it to
(hd0) /dev/sdb
(hd1) /dev/sda
menu.lst only contains hd0,0 for Mageia (and hd1 for Mandriva 2010.2 on the internal harddisk)
But even with this change I can not boot from the external USB disk: same issue.
I could install Mageia 1 beta 1 x86_64 on another external USB disk, but then I booted from DVD and installed grub on /dev/sdb1 and that works fine. But after every kernel update I have to adapt /boot/grub/device.map and menu.lst.