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?

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

I can't replicate this problem. IOW, SUM(INDIRECT.EXT(..)) returns
expected results whatever the starting and ending rows are. Are you
sure you're using the latest version of MOREFUNC.XLL? The current
version is 4.2 released 5 March 2006.