FYI - I found a small utility called Wake Assist. It seems there is a bug with wake up on some Macs. The utility provides a mouse click after it wakes up. Someone even wrote a feedback post where the poster mentions that they were having trouble with scheduled backups with OS 10.4.8:
http://www.versiontracker.com/php/fe...70209112042353
I am giving it a try and I thought this may be useful.