[CentOS] server fails reboot - GRUB

Hi All,
Just wondering if anyone else has experienced a reboot problem.
I have a intel motherboard based server of late 2004 vintage with 2 by 
160GB SATA drives running linux software raid 1 with boot partition and /.
The server was running 2.6.18-128.1.10 and had uptime of approx 40 days.
I did a yum update and reboot yesterday and after the POST the screen 
showed
GRUB

 From memory this indicates that the mbr is corrupt and grub cannot find 
the required files on /boot/grub (the first partition).
I did a linux rescue from DVD which sets the HW raid 5 as /dev/sda and 
the two OS drives as sdb and sdc
then ran grub and did
grub> root(hd1,1) # designates that the root is to be found on the 
second partition of the second drive
grub> setup(hd1) #writes the mbr onto the second drive (/dev/sdb)
grub> root(hd2,1) # designates that the root is to be found on the 
second partition of the third drive
grub> setup(hd2) #writes the mbr onto the third drive (/dev/sdc)
grub> quit
reboot
ALL was well.
Question is, what has happened in the last 40 days to the mbr(s)??
Not sure where to start looking.
Any thoughts
Thanks
Rob
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