I think Asim actually keeps its mountlists in SYS:Storage/DOSDrivers and then mounts them with a command in user-startup. It's because an AsimCDFS: assign is needed but isn't available at the time the contents of Devs:DOSDrivers are mounted in the startup-sequence.
Anyway, the SCSIInquire tool on the AsimCDFS install disk can tell you the device and unit settings for your CD drive(s).
