debian 6.0.4 grub installation failed.

[Muhammad Yousuf Khan <sirtcp@gmail.com>]

i just bought 2TBx2 Harddrives and a computer for making it a
centralized archiving server. moving forward everything went well but
during the installation in RAID creating i noticed that i could not
change the "boot flag " to "on"  it was off and i hit several enters
but it stays off. so i just continued with off. now when installation
completed . setup stops at grup installation and throw an error
"grub-install /dev/sda failed its a fatal error "

any idea why this?

i never face this type of error before. please help.

Thanks