Booting Ubuntu12.04 in recovery mode ,I
managed to boot with the GT630 -4GB graphics card performing as intended.
Later I found in Systems >>>Additional drivers : nvidia accelerated driver
,version ´experimental 310´ when following installation I could resume
´normal booting´ . So this minor problem is now solved.
However I still don´t understand why ,prior to installing ´experimental
driver 310´ , the graphics card performed as intended when booting in
recovery mode .