Where 'was' the OS originally installed to? Was it one of the SATA hard drives? I assume it was never installed anywhere else and you added 2 SATA drives?
Check your BIOS? I have a DELL and it is a different way than some other BIOS in determining the boot order. I have to make changes in TWO places. One is the order of boot, but it is done in GROUPS, that is REMOVABLE DEVICES are set first and HARD DRIVES second in my case. Then I have to go to the GROUP settings and select devices and drives to be included and in what order. You might need to do the same?
In most BIOS's you do have a boot order, you set up your order by moving the devices around in the list and then save the settings and it works.
One thing to try, since the SATA drives will be storage, remove them from the BOOT ORDER, see if that works?