With the SCSI Nikon I tried hooking up via an Adaptec PCMCIA to SCSI adapter I learned that the Nikon specifically needs to be connected directly to a PCI SCSI card. USB to legacy adapters tend to be flakey anway.
On e-bay you'll notice that Nikon USB equipped scanners now command serious prices while SCSI ones don't -- blame Apple for abandoning SCSI.
If you can, I would add a PCI Adaptec card to your computer.
Otherwise, you might look at something like this: http://www.jessops.com/online.store/products/76664/show.html