I currently have an iMac and am interested in using Time Machine to back up my system. I'm intersted in using an ext hd rather than buying the Time Capsule. I have a couple of questions and I'm hoping some experienced people can help answer them. So I currently have my computer set to turn off the screen after 10 minutes on non-activity and to go into sleep mode after 1 hour of no activity. 1. How will an ext hd set up with TM work with this set up? a. I understand that if the computer goes to sleep, TM will not run as it's basically a program and programs don't run when the omputer is sleeping. 2. Does TM only run a back up (after very first back up is made) when files are changed? a. The advertising material says it runs every hour, every day, every week, every month... 3. One of my main question is, if TM runs every hour, every day, etc..., I don't necessarily want TM running all the time and prefer the computer to go into sleep mode so it isn't backing up my computer when no activity has taken place such as when I'm asleep at 2am. a. So for example, if I use the computer at 8pm and do some work with the ext hd attached via thunderbolt, I assume at some time, TM will begin backing up to the ext HD. Let's say I finish on the computer at 9pm and don't use it for the rest of the night. Based on my settings, at 9:10pm, the screen turns off and at 10pm, the computer goes into sleep mode. Remember, this is with the ext hd attached. Will the system stay asleep until I wake it up? b. ...or will TM prevent my computer going to sleep because it's trying to complete a back up every hour, every day, every week, etc...? c. When I do wake it up, will TM begin a back up automatically?