Why should a machine not boot liveCD if HD fails?

[mike cloaked]

In the past few days as some will know from a previous thread I had a total
failure of a machine that is now resolved after replacing the hard drive -
however one thing puzzles me - why would a computer with a failed SATA hard
drive be unable to boot from a liveDVD in the optical drive?

Maybe this is a suitable mystery to solve as we come to Halloween!