questions about Time Machine

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?

Thanks very much for your guys' help. It does answer my questions so I now have a better understanding. I'll be picking up an ext hdd soon. Right now, I'm leaning towards the Seagate GoFlex fr Mac Ultra-portable with thunderbolt or the GoFlex Desk for Mac with thunderbolt. I haven't decided which one although the portable is a max of 1tb whereas the desk is showing a 3tb version for about $250 more than the portable drive. My iMac is only 1tb so I might just stick with the portable and plus this way, I don't have to plug the drive into a powersource as it appears that the portable drive can be powered through the thunderbolt cable without a separate cable for power.

AcesHigh87 said:

Might I ask a question. If this drive is purely for Time Machine why spend the extra money on a thunderbolt drive? TB is lightning fast, no doubt about it, but it's designed that way to be used as like a video editing drive or boot drive or something similar. Time Machine runs fast no matter what so I'd say save your money and just use a FW800 or USB drive.

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I'm not sure if I agree with you saying that it was designed that way. Thunderbolt is just another connection that is superfast compared to the other connections and it's to be used for whatever device can connect to it. In this case, it's a perfect match because backing up data can come in any form from a little bit of data to a very heavy load and having that data transferred via thunderbolt is a better use of the computer's resources since it can be completed in a much shorter time.

AcesHigh87 said:

If it wasn't designed for that purpose it has become it's main function. Point is that having that kind of transfer speeds for backup files is insane and a waste of money. Sure, if you've got $700 burning a hole in your pocket it's a great idea but for $200 or less you can get a USB or Firewire drive that will be more than fast enough for time machine purposes. My USB 2 drive ran the first TM backup (aka the longest one by a mile) in under an hour and completes them now in under 5 minutes. FW 800 would be even faster than that. If somehow 5 minutes is too slow for you, yeah, feel free to spend that kind of money on a thunderbolt drive you don't need.

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I hear you for $700 and that's why I'm not looking at anything in that price range. Like I said in my earlier post, I did find a $200 1tb drive with a thunderbolt adaptor that will also work with another existing portable drive from seagate that I currently own so that one seems to work for me.