Description of problem:
Fresh installed but unbootable system [1] failed with some message that  partition could not be found,
so I booted the installer again (on USB stick) and chose rescue.

Here are the last few lines
(manually typed down from a bad focused phone photo, so may not be precise)

-----------------8<----------------------
running: sh /boot/grub2/install.sh with root /mnt
grub2-install failed:
()
Error: (grub2-install: error: /usr/lib/grub/x86_64-efi/modinfo.sh doesn't exist. Please specify --target or --directory.
>
grub2-install failed: grub2-install: error: /usr/lib/grub/x86_64-efi/modinfo.sh doesn't exist. Please specify --target or --directory.
(press Enter to return to Rescue menu)_
----------------->8----------------------
(yes error is displayed double)


==========================================================================
[1] how the broken system got installed:
64 bit sta1 DVD iso put by dd on a USB stick, booted in non uefi mode
partitionned manually;
first a ext 4 /boot.
then three partiitons used by a LVM
in that lvm: swap, /, /home
When there pressing next it refused and said something like it need /boot to be bootable.  Have never seen that before.  Tried by formattign /boot but same result.  It say in the desciption section below the partiiton graphics that the partiton is bootable.  Just for trying something i pressed teh button to partition automatically and then it added a very small partition in the LVM, but did not assign a mount point for it.  Weird.  But nor it could proceed and everything looked like it went OK, until reboot.
There it said it could not find a UUID.  I checked and there are no such partition ID in the system, that it was looking for...

Stupidly i did not retrieve the install log, sorry.

As i had to get it going quickly (not to make my son disappointed ;) ) , i reinstalled and this time let it partition automatically ( /, swap, /home) and that way it can boot and run OK.

So as a side note there is some problem when installing when LVM is used, and all partitions are defined using the mga6 installer.  A contributing cause may be that on sdb there is SteamOS installed (separately, and all sda is for mageia).

Sidenote: Mageia installer configured grub2 properly to be able to boot Steamos too :)