Kernel panic booting CentOS 5 image in VirtualBox

I'm using VirtualBox, and have a vdi. It works fine on the virtual machine I originally configured for it, but if I clone it (with VBoxManage clonehd) or just copy it, create a new VM and boot it, I always get the same errors:

mount: could not find filesystem '/dev/root'
...
Kernel panic - not syncing: Attempted to kill init!

Anyone know how I can fix this?

The VDI is an image of a clean system that I want to keep reusing to save me having to keep installing the OS into a new empty VDI.

The problem is the layout of the hard disks changed. This is probably because the .vdi hard disk is now attached to the sata controller instead of the ide controller. just disconnect from the sata controller in the settings and reconnect to the ide controller then reboot.