Newbie Q - do I need another external HD?

[rhkmail]

Hi,
I am relatively new Mac user, have been using Time Machine, and just learned about SD. I am going to buy it. My question is whether I need to buy another (3rd) external hard drive.

Here is what I have now:
1st external hard drive - time machine backups (of files on my MacBook Pro)
2nd external hard drive - raw video footage and photos

First I need to figure out how to back up the 2nd external hard drive to Time Machine since those files aren't on my MacBook. Then I'm going to use SD to back up everything.

When I get SD, can I put the SD backup on the 1st HD that already has Time Machine backups on it? I didn't do any partitioning or anything.

Or do I need a 3rd HD?

Also, will I be able to make a bootable SD backup that includes files on my Laptop and files on the 2nd external HD (videos, etc.)

I'm so glad there's a user's manual, hopefully this is covered, but I want to know this is possible (and whether I need another HD) before I buy it.

Thanks so much for any advice on this.