Hi. I have the same issue. Just got my F4-423 today.
I have 4 drives as TRaid and wanted to use my two 500GB SSD as Hyperache. But as soon as I create the storage pool the option of creating a Hypercache gets disabled.
Running TOS 5.0.176
I can´t even create another pool with just the two SSDs. Do I need to wait until Raid sync is finished?