If you want to maintain the Windows-managed dual boot, use the more advanced EasyBCD 2.0 beta (after doing quick forum registration) to remove and then Add back XP.
If you want to boot via BIOS as I described earlier, then unplug Win7 HD which already is System Active, set XP HD as first to boot in BIOS, and run a Repair Install of XP by booting CD, continuing as if to install until it discovers the installation, then choosing "R" to repair the highlighted XP installation.
This will reassert the XP bootloader onto its partition and make it fully independent of Win7 HD when you plug it back in.
Set the preferred HD to boot first in BIOS setup, then to boot the other use the Fkey given on first boot screen for one-time Boot Menu to choose it. This keeps both HD's independent so they can come and go as you please.