I have an external scsi media disk array with a scsi card in a pci slot on my win xp pro system as a non-system drive. I was able to install the scsi card from inside xp like ya do with any other pci card. Then I turned on my disk and did a re-fresh in computer devices and viola ...
So I'm not *NOT SURE*, but you may not need to do the load scsi drivers when you install xp unless the drive is for system stuff.
You said that the system sees your drive but you can't assign a letter, right? That goes to what someone posted earlier -- that you need to partition it.
you might try using partition magic software. It lets you do a bunch of stuff a lot easier (and probably a lot more solid) than just computer management.
Also, if it will only let you make the disk a dynamic disk, does it say it will delete the data when it does that, or is making dynamic an option?
also, just a thought - how BIG is the scsi and is it internal or external? At one point I had a a hitch with installing external drives over 120 gig -- It might have been my usb 2 enclosures, but win xp didn't recognize them beyond 120 gig.
deb