Time Machine not starting when I connect the HDD

Question

I have a 500 GB WD My Passport with two partitions on it, one of which is set as a Time Machine backup drive. The problem is that the backup doesn't start automatically when I connect the HDD. Any idea why?
If it helps, the only thing I did was to partition it, I haven't formatted it, so the SmartWare drive kicks in every time I connect it.

Environment: Last generation MacBook Pro; Mac OS X 10.6.8.