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Without having the XP drive plugged in the 7 boot files, BCD store, and mbr entries were placed on the sata drive by default. To see 7 startup without going into the bios look for the boot device menu if available often the F1, F8, or F12 key is pressed right after the first post screen appears and the lights on the keyboard flash for the first time. You then select the sata drive from the hard drive listing there.
For seeing XP added into the 7 BCD you will first need to go into the menu bar>folder options>view tab in any Windows Explorer window to uncheck the hide protected operating system files and files with known extensions items there. Once visible while browsing you will need to copy the XP boot files NT Detect.com, ntldr, and a copy of the boot.ini file over to the root of the 7 drive.
The next step once the files are present on the 7 root directory would start off with a review on the tutorial on how to go about setting up a dual in the second part. Dual Boot Installation with Windows 7 and XP
The tool for seeing the XP entry called EasyBCD is available at Download EasyBCD 1.7.2 - NeoSmart Technologies
with the general documentation seen at EasyBCD Documentation Home - NeoSmart Technologies Wiki
Once you have the XP entry successfully added into the 7 boot loader you will need to go into the bios then in order to change the default boot device to the sata drive in the boot order section found there. You still keep XP as the default OS however in the system>advanced system settings>startup and recovery section in 7 or made easier by the setting section in the EasyBCD tool itself.