Drive letter assignment on dual boot of two Win7 installs

I need to have have two copies of my Win7 install on the same harddrive, because one is slightly different than the other for testing purposes.  However, one version is always the C drive and the other is always the D drive, no matter which one I boot into.  I need to make it so whichever install is booted into appears as the C drive.  How do I do this?

I must admit, I took a shortcut.  After I got the first install done with everything I needed, for the second partition I installed the very minimum just to set up the boot records et al correctly.  Then I copied the partition image from the first install over this partition so I could just make the mods necessary for the second one.  This may be the cause of my problem, and I have to install and load the second partition from scratch and just duplicate all my previous effort, but I'm not sure and I really hope I don't to do all this extra work.

Is there any mods I can make to correct this issue?  Or is there something else I can do?  From what I've read on the internet, there are people out there with dual boots that both appear as the C drive when booted into.
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noxchoConnect With a Mentor Global Support CoordinatorCommented:
Follow the instructions given in this article:
It will be always C: drive.
laffmanAuthor Commented:
I guess I should add that the hard drive partitions look like this:

1)  System Partition
2)  Main Boot
3)  Data Drive
4)  Second Boot

and they are all primary partitions
laffmanAuthor Commented:
When running easy bcdedit, at one point I could swear the second boot used a guid to identify the partition, instead of a drive letter.  Is that the trick, having them both point to the partition guid instead of a drive letter?  But how do I change that, I tried command line bcdedit and that didn't take the guid as a value.
David Johnson, CD, MVPOwnerCommented:
you are correct in that you have to start from scratch, do a base install then robocopy your last backup overtop (mount the larger .vhd using diskpart)
laffmanAuthor Commented:
After reading this post, I realized the important points were (1) hiding the partitions that I didn't want driver letters automatically assigned to, and (2) install everything needed from scratch instead of copying the image over and modifying it.  Since I already had a bootable install on partition 4, I booted into it and did the hiding from there, so I didn't need any 3rd party partition tool, I just used to the installer to handle my partitions.

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