So you’ve taken your physical install of centos/rhel 5 and cloned the disk into a virtual machine, and now it won’t boot, or even mor eannoyingly it half boots and then tells you it can’t find it’s partitions even though it’s just happily read it’s way through boot.
The usual touted fix for this is to make sure your disk is set to IDE, and this does work, but it’s an annoying fix, so to work as a scsi disk:
edit /etc/modprobe.conf and remove any alias lines referring to scsi_hostadapter and replace with
alias scsi_hostadapter1 mptspi
alias scsi_hostadapter 2 ata_pixx
then we need to rebuild the initrd so these modules are available at boot
and done; sync && reboot