I took ABEKL advice and flash the Bios to version F7. I was a bit reluctant to update to the new Bios via the Internet but in the end I bit the bullet and did it anyway, actually it is quick and painless, lots better than the old "floppy way"
Once the computer reboots with new BIOS 24 GB is instantly recognized, Windows has no problem using it. Yes it is W7 64 bits Ultimate.
Thank you everyone for your very quick response, your helps are greatly appreciated