Hi, According to the manual:
2 xExternal SATA 150/300 (SATA On-the-G0), supporting SATA RAID 0, 1, 10, and 5**
** A port-multiplier (excluded in package) is required while using RAID 5 or 10 on eSATA ports.
On the other hand, 88SE6121 does support port multiplier.
I would try firstly the e-SATA port and if not working, buy a PCI controller that does support port multiplier. Something like http://eu.startech.com/Cards-Adapters/HDD-Controllers/SATA-Cards/2-Port-PCI-Express-eSATA-Controller-Card