My 160gb and 500gb are

Question

My 160gb and 500gb are not showing on the desk top last night they were there. I hasve done all the simple trouble shooting turning htrem off and on, unpluging the power and firewire cords and rebooting the computer nothing seems to work. I can hear the disk spinning when the computer boots up but hrey do not mount on the desk top.They are connected to a mirror door G4 and have been working fine until now.Hope you can help Thanks Bob Black