Yes, as you suspect both drives are needed to boot into either Vista or Windows 7. In order to get your 1.5 TB WD drive set up as bootable you have to use tools outside of the Windows operating system, i.e. you have to boot from a CD into disk management software.
There is a special version of Acronis software that WD makes available here:
Make sure to read the manual:
There is also this free tool:
Focus on the guides here that talk about fixing the boot record.
A quick overview of what you will be doing:
Create a bootable CD (either Acronis or Lazesoft - actually might as well create both)
Go into your BIOS setup and set the boot order so that CDs are looked at before hard drives
Install the WD with your existing Disk0.
Boot off of CD
Copy all partitions from Disk0 to WD (keep the same size on target for now)
Swap Disk1 for Disk0
Boot off of CD and copy all Disk1 partitions to WD
At this point you have all the bits and bytes from your two drives on the WD. Take Disk1 out of the system so ONLY the WD is installed.
Reread the manuals about restoring the MBR and making Windows drives bootable.
Make sure ONLY the WD drive is in the box when running MBR recovery tools - if you inadvertantly had either Disk0 or Disk1 installed it could render them unbootable!
Boot off of one of the CDs. Try the MBR and boot sector recovery tools. See if it works. If it doesn’t, boot off the other CD, try its recovery tool.
The nice thing about this is that your original drives and data are not in any jeopordy. Regardless of whether you succeed or fail at getting the WD drive bootable you can always put the old drives back in and be back to where you started.
One other thought - and I don’t mean to discourage you, but…
Several years ago I had a Dell 4600 that I couldn’t clone a bootable drive for. At the time I was an IT “professional” and had cloned and set up hundreds of desktops and laptops at work, so I had plenty of experience. So don’t be discouraged if you are not successful.
And a final thought: If you can’t get the WD to boot into Windows (or don’t even want to try) check out your motherboard and see if it has any empty SATA sockets. If it does you can install a bracket to give you an eSATA socket on the back of your machine. Then put your WD in an eSATA enclosure. That’s what I wound up doing with my Dell 4600 (I was quite surprised that it had the open sockets.)
I used this:
http://www.amazon.com/eSATA-Bracket-Power-External-Cable/dp/B000YI7M3G and put one in all my subsequent machines.
And this enclosure: