You cannot post complete code even though you have posted pieces?
I just created xlsb file, seems can manipulate same as xlsx.
The following simple example works for me.
Code:
Dim xl As Excel.Application, xlw1 As Excel.Workbook, xlw2 As Excel.Workbook
Dim strWB As String
strWB = "Condos.xlsb"
Set xl = CreateObject("Excel.Application")
Set xlw1 = xl.Workbooks.Open("C:\Users\June\Condos.xlsx")
Set xlw2 = xl.Workbooks.Open("C:\Users\June\" & strWB)
xlw1.Sheets(1).Copy After:=xlw2.Sheets(xlw2.Sheets.Count)