Hi Jim. Smart to boot two OS HD's via BIOS so they can come and go as you please. Unfortunately you didn't know that Windows 7 will configure a multi-boot if installed while another OS is plugged in.
Can you please post back a screenshot of your full Disk Management drive map and listings, using Snipping Tool in Start Menu? There may be something unmentioned about your setup that needs to be taken into account before we can give you failsafe steps.
If XP alone is marked System Active then the procedure is to mark Windows 7 Active in XP Disk Management, power down, unplug XP HD, set Windows 7 HD as first HD to boot in BIOS setup (after DVD drive) then boot the Windows 7 DVD Repair console or 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. System Repair Disc - Create Startup Repair - Run 3 Separate Times
When you plug XP HD back in set preferred HD as first to boot in BIOS setup, then boot the other HD by tapping one-time BIOS Boot Menu during bootup:
Asus - F8
HP/Compaq - Esc
Sony - F2
Acer - F2
Gateway - F10
eMachnes - F10
Toshiba - F12
Dell - F12
IBM/Lenovo - the blue Thinkvantage button
If the Dual Boot menu still appears when booting XP, remove it by installing EasyBCD 2.0 to Edit Boot menu. You'll need to install Net Framework 2.0 to use EasyBCD in XP. Welcome to EasyBCD 2.0! — The NeoSmart Files