This bit of advice is offered in
http://linuxusbguide.sourceforge.net/USB-guide-1.0.9/x485.html
If it isn't, you will need to type the following, as root:
echo "scsi add-single-device 1 0 0 0" > /proc/scsi/scsi
There is no explanation of what that is, what it should
do, and how you can tell if it worked or not, what
paramaters you might need to try and so on.
Also, mass storage devices are also things like
flash/smartmedia drivers, so it's hard to know what
type of device you might want to pretend to be.