Boot error "Block device /dev/sdaX is not a valid root device"

Problem Description

Linux fails to boot

Symptoms / Error messages

at this point the kernel has loaded but can't mount the root partition

console errors

Activating mdev
Block device /dev/sdaX is not a valid root device...
Count not find the roo block device in .

at the shell prompt, list of /dev shows there are NO sd devices

ls /dev/sd*

Kernel lacks correct drivers for the boot / hard drive controller.

recompile the linux kernel with the appropriate drivers.  Hardware can be determined with utilities such as lsusb & lspci run from a boot cd / usb.