Quote: However, the scratch disk option in CS3 only allows me to select a fixed drive.
For the very good reason that an ejectable drive needs to be ejected - if a drive containing active scratch space goes off line the application is certain to crash.
Try looking for a hack to make your PCI Express card appear to be a fixed drive.
Are you sure that the slowness is because of scratch file accesses? It could just be that working with five 220MB image files at the same time is moving around a hell of a lot of data....