Update
Worth noting that the Firewire External hard disk (partitioned into Time Machine and Other) were present in the Sidebar before the Update.
Also worth noting that after the update the disks didn't appear in Disk Utility.
Apple System Profiler (ASP) showed that the Firewire was connected to an unrecognisable device ?
I then tried swapping other Firewire cables and the other Firewire port on the drive - all of these showed up in the ASP as being connected to an unrecognisable device !
But for the sake of the likely few remaining Snow Leopard users that may strike this issue the Mini USB port on the external drive, happily brought back the external drive, and thus my Time Machine, eureka !
Nevertheless I'm now left pondering whether Apple has dropped support and connectivity to Firewire ports and devices, or whether this issue pertains solely to my machine ?