Flash Mode is a great feature ensuring that VM workloads remain within the flash (SSD) tier of storage. Once flash mode is enabled for a virtual machine, all of the disks associated with that VM (including any future created disks) automatically get added to the flash tier.
However, sometimes having so many disks within the flash tier can cause performance degradation for other VMs that are not configured for flash mode (but could benefit from using the flash tier of storage) or can cause the available flash tier space to be consumed too quickly. Further, it is sometimes not desirable to have all of the disks associated with a virtual machine contained within the flash tier.
Accordingly and, though not available as a Prism Web user-interface (UI) modifiable option, individual VM disks can be configured to not use the flash tier even while the VM itself is configured for Flash Mode. The procedure for removing individual VM disks from the flash tier involves using the Acropolis Command-Line Interface (ACLI) while connected to one of the controller VMs (CVMs) of the cluster via secure shell (SSH). You can find more information regarding Flash Mode, along with the procedure to remove individual disks, from the Flash Mode for Virtual Machines and Volume Groups section of the Prism Web Console Guide.