Sheesh! Found it.

Select Appointment, go to "Edit" and select "Copy", then move to the
appropriate other calendar and (making sure you are in the right time slot)
go to "Edit" and select "Paste".

OR

Select the Appointment and hold down the "CTRL" key while dragging which
will "Copy" the appointment across.

Again the key is to make sure you drop it in the correct time slot as the
time will change to accomodate where you put it, it won't retain the original
time information.

Thanks for helping me think it through.