[CentOS] i can't install centos on my poweredge 2950

hi

i m trying to install centos5 on my poweredge 2950 with no luck.
the raid controler is
LSI Logic / Symbios Logic MegaRAID SAS 1078 (rev 04)

indeed all is going fine during the installation but when i reboot the 
server panic

the first error i got is this one
error mounting /dev/root on /sysroot as ext3: block device required

it seems that it can't find the /dev/root devices.


but when i boot in rescue mode the / partition is found without any problem.
i can chroot in it

i have tried with 5.5 and 5.4 x86_64 but i always got the problem.

i m really stuck with that :(

Does someone has an idea of what could happened?