I would dual boot via the BIOS so that the drive can come and go as you please and not be intertwined with XP HD. Using this method, you set the preferred HD to boot first in BIOS setup, then if you want to boot the other HD/OS you trigger it instead using the one-time Boot Menu F-key given on first boot screen.
To set up your Dual Boot this way, unplug the XP HD on target machine, plug in Windows 7 HD, make sure it is set as first HD to boot in BIOS setup (after DVD drive).
Now boot the Windows 7 DVD/Repair CD, press Shift F10 to open a Command Line, type:
SELECT DISK 0 (confirm from list this is Windows 7 HD #)
SELECT PARTITION # (replace # with Windows 7 partition # from list)
Now boot back into DVD Repair console/Repair CD, click through to Recovery Tools list to run Startup Repair up to 3 separate times with reboots to write the MBR to Windows 7.
When Windows 7 starts, shut down and plug in XP HD. Change boot order if you want, or trigger XP HD using one-time F-key given on first bootup screen for Boot Menu.
If you return Windows 7 to other machine it will boot the same way via BIOS now.