Not sure this is the best title for this question but here goes.

Through python/Qt I started multiple processes of an executable. Each process is writing a large file (~20GB) to disk in chunks. I am finding that the first process to start is always the last to finish and continues on much, much longer than the other processes (despite having the same amount of data to write). 

Performance monitors show that the process is still using the expected amount of RAM (~1GB), but the disk activity from the process has slowed to a trickle. 

Why would this happen? It is as though the first process started somehow gets its' disk access 'blocked' by the other processes and then doesnt recover after the other processes have finished...

Would the OS (windows) be causing this? What can I do to alleviate this?