Check your PC0 mountlist in case it has been modified. To do this, open up a shell and type in "type devs:dosdrivers/pc0". This will show you the PC0 mountlist. Also, go to the DEVS:DOSDrivers drawer on your Workbench disk, click once on the PC0 icon, go to the menu and choose "Information" and check the unit tooltype. Post the results here so we can check them for you.
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moto