If you have a mounlist in SYS:Storage/DOSDrivers, then mount the mountlist using for example mount cd0: without specifying the path, then it will find the mountlist in SYS:Storage/DOSDrivers.
If you add an Assign to SYS to another volume which also contains a Storage/DOSDrivers drawer with mountlists in it, then these will also be found by the mount command.
So I have to disagree Piru - I think it is working because of the added SYS: assign.
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moto