Time Machine wants to back up everything

I have just purchased an external hard drive, connected it to my Mac and used Carbon Copy Cloner to create a complete bootable copy of my entire internal hard drive. I then switched on Time Machine, which began to back up my hard drive and told me that it would take about 3 hours to do so.

1. I understood that Time Machine would only back up any changes that I have made since creating the copy of my internal HD. Why is it trying to back up the whole?

2. I do not keep my computer running except when I am actually using it. I also do not wish to have my external HD running every time I start up my computer, because I wish to minimize the risk of the external HD deteriorating and eventually failing with time. What will Time Machine do in these circumstances?

3. I understand that Time Machine will keep earlier versions of my backed-up HD as well as the latest version. If my internal HD should fail and I should have to use the external drive to boot up my computer, which of the saved versions will be available for booting up?

Thank you in anticipation of replies to these three questions.