Filling in blank cells with values from another workbook

Veraborn64 

This outlines an approach that load values to an from the workbook(s) as a block and sorts the data transfer between arrays held in memory.

Option Explicit
Option Base 1

Sub transfer()
Dim vSource
Dim vDestination
Dim i As Long, j As Long
Dim v
'   Load Source and Destination ranges into variant arrays
    vSource = [Source].Value
    vDestination = [Destination].Value
'   Loop over elements of array copying values to empty locations
    For i = 1 To UBound(vDestination, 1)
        For j = 1 To UBound(vDestination, 2)
            v = vDestination(i, j)
            If v = vbEmpty Then _
                vDestination(i, j) = vSource(i, j)
        Next j
    Next i
 '  Return augmented array to destination range
    [Destination].Value = vDestination
End Sub