You've already found a good solution that got you to where you need to be. So this is more for others who are following for their own interest. What OP did not realize (because the supplier did things without telling OP) is that, once the BIOS was set to use the two drive in a RAID mode, then there is a whole other step using other built-in utilities to actually manage the RAID system.
As a system is set up, if the BIOS is set to use RAID mode for the SATA drives connected to it, this triggers about three details. One is that it means you are planning specifically to use the mobo chipset's built-in ability to manage RAID arrays (that is, you are not going to use a separate add-on RAID controller card); the Windows OS will have to have a RAID driver added to it as part of the early installation process (OP did not have this issue since it was done by his supplier); and, you MUST use the built-in tools to set up and manage the RAID array, and there is a specific key to access this utility set.
If the BIOS is set to use RAID mode, you will find early in the boot process that the POST screens include a prompt to press a specific key (maybe CTRL-I, maybe something else) to enter the RAID Setup screens. (If you do nothing, this will time out and proceed anyway.) If you do press this key, you enter a whole set of tools to examine what RAID arrays and drive units exist, to create RAID arrays of various types and configure them, to specify which disk units are assigned to which RAID array and which are NOT involved at all in any RAID array, to diagnose and possibly repair problems with the arrays, and to break up the arrays into separate drives again. There are detailed manuals for all this - sometimes included in your mobo manual, but more often as a large separate document. In OP's case, what he COULD have done had he known this, is to use those tools to break up the RAID0 array into two empty drive units that were ready to be Partitioned and Formatted just like any other brand new empty HDD unit, and ready for installation of Windows on one of them.
By the way, JBOD is not really part of the RAID versions, and technically there are other better names for it like Spanning. It's just that many chipset and BIOS makers who include some RAID ability in the mobo also include JBOD as an option because it's not very complicated and hence easy to add in as a feature.