When you had the single drive with Windows XP, you have some partitioning scheme setup on that single hard disk. Let's for simplicity call it a single partition. That partition was the primary one, and it was marked "active".
When you setup the new drive, I'm guessing that you also made it a single, primary partition ... which tends to be made "active" also.
OR, the other drive's partitioning was secondary, which means it was not marked "active".
So, when you swapped the drives, either both were "active" and caused a problem, or your "boot" drive wasn't actually bootable (it having a secondary partition) and thus caused the need to boot from the old drive anyway.
What I'm trying to say is, you probably have a partitioning problem that is easy enough to fix once we define what it really is. By using FDISK and using its option #4, you can examine the partitions on your drives and can let us know more precisely how it looks.