Changing XP to different hardware requires a Repair install from boot.
Unplug the Windows 7 HD, plug in XP internally (not externally) but DO NOT START IT. Instead boot from XP CD used for original install (or same version) and follow these steps: Repair Windows XP - How to Perform a Repair Installation of Windows XP - Part 1 of 2
If after discovering the XP installation, the booted CD does not present an "R" for repair option then it cannot be repaired to run on that hardware and needs clean install.
If it completes the Repair to acclimate XP to the new hardware, then you have the choice to plug back in Windows 7 and boot from the BIOS your preferred HD first, or interrupt preferred HD booting by tapping the key given on first BIOS screen for Boot Menu to trigger the other HD.
Having a BIOS-managed Dual Boot makes the HD's independent so they can come and go as you please. If you wish to instead create a Windows-managed Dual Boot menu, you can do so from Windows 7 using EasyBCD 2.0 beta
by adding XP, however this makes the HD's interdependent and harder to remove.