Assistance with issue after switching from 32-bit to 64-bit

[David H]

Hi all, I'm having an unusual problem where I can't seem to get grub,
or the OS, to load if I take a previously used set of drives that had
a 32-bit flavor of CentOS/RHEL on them and perform a fresh install of
64-bit CentOS/RHEL.  I've run into the same issue on both Dell and HP
servers.  At first I thought it was the bios blocking the new boot
record from being written out but I've since taken the physical drives
out, put them in a usb external enclosure and used the old 'dd
if=/dev/zero of=/dev/sda bs=512 count=1' to supposedly zero the mbr.
A backup of the MBR before and after shows the file changing so I
believe the command was successful.  parted and fdisk both confirm the
loss of all existing partitions.  After any of these attempts though,
all I'm left with is a flashing cursor as soon as the bios would have
normally called grub.

Has anyone run into this?

Thanks,

David