Raid-1 booting problem

Hi,

I installed a Raid 1 linux system on our server, using the RedHat 7.2 
utility raid system.
Everything is mirrored on this disk (/, /boot, /usr, /tmp..)

I need to change one of these disks (the ibm is not working anymore). I 
partitioned a new disk (exactly the same as the original) and then plugged 
it in the computer. I used raidhotadd /dev/mdx /dev/hdcx to add the second 
 disk. Everything worked fine, but if I unplugged the first disk I am not 
able to boot anymore on the second, I guess there is a problem with lilo 
trying to write on the MBR.

I didn't find anything about it on the HowTos.. out of date. Can someone 
tells me how to make the second disk bootable ?

Thanks,

-Stephan