Hello chchenss,
On-board RAID cards are not supported by ESXi. Not yet anyway. In a case of separate hard disk showing up, that is strange, because when I was playing around with that, nothing would show up at all. Either there are specific ports for RAID setups that your hard-drives have to be connected or somehow RAID did not take effect at all. ESXi.
Either way, getting a cheap RAID card rather than using the on-board one will benefit you a lot more.