This article explains how to add an External Mount to a Virtual appliance to grow beyond the 250 GB and 950 GB OVA templates provided on MySonicWall.com. Once the external mount is added to the Virtual Appliance, the flows will be written to the New mount.
CAUTION: The old flow data that was stored on the flow server will be lost as the data will be written in the new mount after this.
NOTE: This process should be used to expand the volume of Flow Server agents For Syslog based server, we need to follow: https://www.sonicwall.com/support/knowledge-base/how-to-expand-the-hard-drive-of-a-virtual-appliance/170502427615315/
5. Input the size of the disk equal to the amount you would like to expand and select Thick Provision Lazy Zeroed then select next

7. Select Finish and turn on the Virtual Machine

After this the flow data will be written to the new mount.