What RAID-controller do you have in your server? I suppose you use some mainboard-builtin fake-RAID controller, which is not a real hardware RAID but relies on special drivers inside the OS. Without such drivers you'll just see regular a JBOD, so in your case of a fake-RAID1 two independent disks:
http://www.servethehome.com/difference-hardware-raid-hbas-software-raid/
https://ata.wiki.kernel.org/index.php/SATA_RAID_FAQ
ESXi does not support such fake-RAIDs and has no software RAID capabilities itself. Your only solution is getting a real (preferably HCL supported) hardware RAID controller.