Installation of W7 upgrade edition on a seperate disk

I have 2 internal disks. On disk 1 (HDD) I have Vista. On disk 2 (SSD) I want to install Windows7. For the W7 installation I would disconnect disk 1 because I do not want a double boot but I use the BIOS boot sequence to switch between the 2 disks. I am wondering whether that is possible with the W7 upgrade edition (the $50 early bird offering). My Vista is an OEM that came with the PC and I only have the Installation/recovery DVDs that I burnt from the recovery partition. What I am wondering is how the W7 upgrade installation would know that I am eligible for the upgrade.

The computer I bought in 2007 - with Vista preinstalled. But the W7 upgrade edition I have ordered a few weeks ago. I think I will get it after Oct. 22.

Well down to whether you live in UK or not...since as I'm aware only the Uk getting FULL edition as replacement for the UPGRADE edition due to a EU issue over browsers etc...everyone else just get UPGRADE when ordering UPGRADE Editions...far I heard.