I assume you're wanting to automatically boot a different hard-drive? If so, it's a real simple fix. I'll take you through the steps using Windows XP Professional. In the example below we will tell the system to automatically boot Windows 7 if the user does not make a choice in 30 seconds.
1. Open My Computer.
2. Open your C:\ drive.
3. At the top, select Tools -> Folder Options (Picture
4. Select "Show Hidden Files and Folders," Uncheck "Hide Extensions for known File Types," Uncheck "Hide Protected Operating System Files." (Picture
5. Locate "boot.ini." (Picture
6. Right-Click boot.ini and go to "Properties." (Picture
7. Uncheck "Read-only" and press OK. (Picture
8. Open boot.ini
9. Find the line corresponding with the system you want to boot automatically every time your system starts. (Picture
10. Place the system's path into the "Default" slot. (Picture
NOTE: Only place the system's path into the default slot, not the boot options and description!
11. Save the file and exit boot.ini.
12. Once again, right-click boot.ini and go to Properties. (Picture
13. Check "Read-only" and press OK. (Picture
14. Restart your system to ensure the changes were made correctly and that your system starts the correct operating system.
CAUTION: Altering the lines below "[operating systems]" can change the way you system starts and might prevent your system from starting! DO NOT ALTER ANY ENTRIES BELOW "[operating systems]" UNLESS YOU ABSOLUTELY KNOW WHAT YOU ARE DOING! Altering the "Default" slot will not harm your system as long as you enter a system path that is described in the "[operating system]" sector.
Altering boot.ini the way we just did tells the system to automatically start Windows 7 if the user does not make a choice in 30 seconds. Additionally, if you want the timer to run for say 1-3 seconds, you can alter the "timeout" value to read something similar to: timeout=2
That way when the system will automatically boot the "default" system in 2 seconds if the user doesn't make a choice. NOTE: Don't forget, you can't alter boot.ini unless "Read-only" is unchecked. Also, don't forget to check it again once you're finished editing boot.ini.
Let me know if this solved your problem, or if you were wondering how to do something else.