Use time machine for backing up a different computer I've been over many faq's and troubelshooting but can't seem to find an answer to this.

I'm currently running my mac from an external HD. My internal got corrupted, I can't repair it with disc utility. So I booted from an older time machine backup, on an external HD which now is my main HD and what I'm running from.

Anyway I want to put everything back the way it was. Meaning erasing my internal and let time machine put back a back up of my current workspace. But when I let it run he backs up the corrupted internal one. It even makes for a total of 500G, and my original HD is only 320.. so I'm making a backup to big to be used to restore later.

How can I change that he backups only the external information? Or maybe I'm wrong and he needs to back up the internal to for things to work since I still use it as my computer (but not my HD). I don't need the content of the original HD to be backed up along with my external one. I just want the workspace I created with a back up on my external to be put back on my erased, internal one..

I'm also worried if I erase my internal HD the booting help progam (C or R) will be gone to.. will it?
To exclude your internal drive from backup, do the following. Go to System Preferences > Time Machine. Click Options... Under the section titled "Exclude these items from backups", click the + button. Select your internal hard drive. This will prevent Time Machine from backing up your internal hard drive.

If your external hard drive is working just like you'd like your internal hard drive to work after you finish fixing it, you can also restore your external volume onto your internal volume. To do this, boot into Recovery Mode (holding R upon boot). Once booted into Recovery Mode, follow the steps in this Apple Support article, under the section Restoring the backup disk image's contents to your internal Mac OS X disk. You can begin with Step 4, as you have a newer operating system than the article was written for.