You'll need to switch on advanced I/O logging for this particular case and then email the logs. All the details should be here: https://support.division-m.com/hc/en-us/articles/333757012755
Have you got an open support ticket? I expect you'll need one if not. To prepare you, these are the questions that were asked of me:
What sort of file sizes are we talking about?
Do you have enough free space on a/each drive(s) in the pool?
Are you able to replicate this easily?
How many drives do you have in the pool, and what is the free space on these drives?