Dual boot not seeing 2nd hard drive

Vista probably wasnt able to identify xp when it was originally installed there is a 3rd party tool to modify the windows vista bootloader in order to add the option. It also will automatically find other versions of windows.

http://neosmart.net/dl.php?id=1

go there and read up. ive used it several times it works perfectly.

If your fluent enough you can also boot with your vista disk and use r to go to recovery console and or command prompt. Use the fixboot and
fixmbr commands to repair and rewrite the bootloader and record. The interesting part is that vista didnt detect xp originally and automatically add it. I would recommend using the fixmbr fixboot method first then use easybcd if you cant get it to work.

When you run those commands it should search your drives and locate the xp installation and add the choice to the boot loader. If not use easybcd