I'm sure someone will correct me if I'm wrong, but I don't think the USB connection would be fast enough to work seriously with audio.
You could always use a USB drive for storage. Keep a partition on the internal drive for recording to, and working with the audio, then shove your files on a USB drive when you need to work on another project.
As Kris mentioned, best to add a larger internal.
SCSI-1 on the Wallstreet (I presume), may just be good enough to work with audio, if you're not wanting to do any serious multi-track work. Would you want to take a risk on an external SCSI drive from ebay? New SCSI drive would be far too expensive.