HI Eric,
As per my understanding, Instead of opening different files and copying data and pasting them to Main excel, you can try to copy all excels data into collection and append write in Main excel at a time without opening Excel visibly.
As many excels are being Opened at a time it may cause issues in writing Data to another Excel.
Try this and let me know if you are having any issue.
Regards,
------------------------------
Yeswa Vaibhav Alwar Nerella
Associate Consultant
Capgemini
Asia/Kolkata
------------------------------