Did you ever run the upgrade advisor
? It is really pretty good and will tell you about any problems that you may face before getting started.
It may even offer solutions or work-arounds.
Have you gone to the Promise
web-site to see if they have a solution to this problem? It has been a long time since I used an external Promise ATA controller but I think they are about as generic as they come. It is possible you could have other issues, such as also having your built-in ata controller still enabled...
Also, another good place for information (besides here) is Windows Vista Community
where they have specific Vista oriented forums for 'hardware and devices' AND 'installation and setup'.
wish you better luck moving forward...