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ReDim Preserve MyFiles(1 To FNum)
try change to
If FNum > 0 Then
Dim Arr
ReDim Arr(1 To FNum, 1 To 2)
For FNum = LBound(MyFiles) To UBound(MyFiles)
Set mybook = Workbooks.Open(FilePath & MyFiles(FNum))
Sheets("FX Historical Data").Activate
RowCount = Range("A1").End(xlDown).Row
Arr(FNum, 1) = Range(Range("A2"), Range("b2").Offset(RowCount - 1))
Sheets("Position Data").Activate
RowCount = Range("A1").End(xlDown).Row
Arr(FNum, 2) = Range(Range("A2"), Range("d2").Offset(RowCount - 1))
ActiveWorkbook.Close
Next FNum
For FNum = LBound(Arr) To UBound(Arr)
Sheets("FX Historical Data").Activate
Range("A2").Resize(UBound(Arr(FNum, 1), 1), UBound(Arr(FNum, 1), 2)) = Arr(FNum, 1)
Sheets("Position Data").Activate
Range("A2").Resize(UBound(Arr(FNum, 2), 1), UBound(Arr(FNum, 2), 2)) = Arr(FNum, 2)
Next
Sheets("Report").Activate
Application.ScreenUpdating = True
Application.Calculation = xlCalculationAutomatic
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