WARNING! These examples may crash your computer and delete all files if executed inappropriately.
Before removing any files, ensure you have a backup of all critical files. I am not responsible for any data loss.
Login to the Server via SSH and run the following command to find the files across all shares:
find /mnt/user/ -type f -name "*.partial"
Then, if everything looks okay, you can run this command to find AND delete them:
tronyx's post in Hidden partial files on array when mover fails due to not enough space was marked as the answer