Indirect function with external sumif

Hi All
I am struggling with making a formula work as all it does is return a #VALUE when I do not have the external workfile it is referencing open.
I am using the INDIRECT.EXT function but it does not seem to help and I can only assume I have the syntax wrong.

The formula is:-

=SUMIF(INDIRECT.EXT("'D:\["&G3&".xlsx]Sheet1'!$b$2:$b$10"),"a",INDIRECT.EXT("'D:\["&G3&".xlsx]Sheet1'!$C$2:$C$10"))

Where G3 has the external file name I wish to reference.
B2:B10 is a column of letters A through to D
C2:C10 is a column of numbers.

The SUMIF simply adds up the total for a specific letter appearing int he list.
Any help would be appreciated.

Thank you

Ok, thanks for the quick response.