You can use the vSphere Web Client to mark storage devices that are not automatically recognized as local flash devices.

When you configure Virtual SAN or set up a virtual flash resource, your storage environment must include local flash devices.

However, ESXi might not recognize certain storage devices as flash devices when their vendors do not support automatic flash device detection. In other cases, certain non-SATA SAS flash devices might not be detected as local. When devices are not recognized as local flash, they are excluded from the list of devices offered for Virtual SAN or virtual flash resource. Marking these devices as local flash makes them available for virtual SAN and virtual flash resource.