Hi
That has been long, long ago...:-)
Do you work with a Workbench disk, not with a harddisk ?
In this case this indicates that your copy of the Workbench disk is corrupt (the part of the disk, where the DiskCopy command is stored).
You should make a new copy of your original Workbench disk or try to repair the current one using a tool like DiskSalv and make a copy of the repaired disk.
Noster