Hi all. This is my first post and need your help.

I have an iMac with a 1TB hard drive. Here's what I want to do:

1. I want to backup my important files from my iMac to the new 1TB Western Digital My Book External Hard Drive (WDHD) that I just got about 30 minutes ago.

2. Then I want to erase my iMac internal hard drive since I have too much stuff on it. I want to "spring clean" and start fresh.

3. After this, I want that cleaned internal drive to start backing up on the WDHD using Time Machine. Basically it will have nothing substantial to back up to start, but will grow over time.

Essentially, I want part of my WDHD to be reserved for those files I back up prior to the hard drive erase and part of the WDHD to be dedicated to Time Machine backup of anything I do AFTER the hard drive erase.

My questions are:

1. How do I do that?

2. How can I insure that the files I back up on the WDHD BEFORE erasing my internal drive won't be lost when I start Time Machine? I'm afraid that Time Machine will look at my newly erased internal drive and say, "Well, I better start backing up this drive. Oh my, I see files that aren't part of this drive anymore. I better start erasing all this stuff on the WDHD since there's nothing on the internal." (If my iMac literally starts talking about it I'll need some serious therapy.)

I want to get this done ASAP so any help you all provide would be very much appreciated!