I have 2 data drives from other devices in my XPS 8500, so I know that should work for sure. A couple things you could try:
1) Enter the BIOS setup menu during bootup and ensure the boot device is set to the OS HDD
2) If #1 above fails, your data drive might be set to be bootable in some way. Transfer all the data from it to another HDD, then completely reformat it as NTFS on the HP PC. Plug it back into the XPS. Hopefully if boots this time, then transfer the data back onto it.