Additional Drives (F:\) being Generated!

I am running Win NT Workstation 4.0 on an NT Server running BackOffice and NT Server 4.0. Throughout the day additional network drives are being intermittently generated on several NT Workstations. (G:,H:,I;,J:…..etc)These drives do not exist on my server. What is causing this? Bad Hub?(am using twisted pair)Bad NIC? Software? This does NOT happen on Win/95 workstations.