I am trying to sum up a range of cells in an unopen, external worksheet.
Using INDIRECT.EXT, I have only been able to do so if the row number of the
beginning cell is less than 100. For example,

=SUM(INDIRECT.EXT("'C:\[MyBook.xls]MySheet!b99:b704"))

will work. However,

=SUM(INDIRECT.EXT("'C:\[MyBook.xls]MySheet!b100:b704"))

will work only for an open worksheet, not a closed one.

Additionally, if the ending cell row is 1000 or more, regardless of the
beginning cell, the formula will not work for a closed workbook.

Does anyone know if this problem can be solved, so that I can use
INDIRECT.EXT with SUM over the full range of a worksheet?