With separate HD's try booting XP using BIOS Boot Menu key. This keeps the drives independent to come and go as you please.
If it fails to boot, unplug Windows 7 HD and run an XP Repair install: http://pcsupport.about.com/od/operat...txprepair1.htm
Afterwards plug back in the Windows 7 HD, set preferred HS as first to boot in BIOS, then boot the other HD using BIOS Boot menu key.
Or in Windows 7, try removing XP listing from EasyBCD 2.0, then Add it back, accept boot files, autocompletes, Save, restart. If this fails, remove XP listing, unplug the drive and do the XP Repair, boot via BIOS.