That does seem pretty weird. Changing the drive via the immediate window affects the GetOpenFileName, but not the SaveAsFileName dialogs...
I'd probably work around the issue with something like:
Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
Dim Mystr As String
Dim var As Variant
Application.EnableEvents = False
If SaveAsUI = True Then
Mystr = "C:\test"
var = Application.GetSaveAsFilename(Mystr)
If Not var = False Then ActiveWorkbook.SaveAs var
Cancel = True
End If
Application.EnableEvents = True
End Sub