I suspect you have a “fake RAID” controller. These RAID controllers only provide minimal support in the BIOS; the bulk of the work is done in a Windows driver. Linux often copes poorly with these controllers because it lacks Windows drivers. Seeing single UUID but two disks is common symptom of fake RAID (the Windows driver would know that there are two disks but they are supposed to have identical content).

If you have a fake RAID device (and you probably do: consumer-grade RAID controllers are almost always fake RAID), and you aren't sharing the disks with Windows, turn off RAID in the BIOS. Then enable Linux's software RAID, which for RAID-1 is superior in all respects (except when you're sharing the disks with Windows).

See also the Ubuntu fake RAID howto, How do I differentiate “fake RAID” from real RAID?

I suspect you have a “fake RAID” controller. These RAID controllers only provide minimal support in the BIOS; the bulk of the work is done in a Windows driver. Linux often copes poorly with these controllers because it lacks Windows drivers. Seeing single UUID but two disks is common symptom of fake RAID (the Windows driver would know that there are two disks but they are supposed to have identical content).

If you have a fake RAID device (and you probably do: consumer-grade RAID controllers are almost always fake RAID), and you aren't sharing the disks with Windows, turn off RAID in the BIOS. Then enable Linux's software RAID, which for RAID-1 is superior in all respects (except when you're sharing the disks with Windows).

See also the Ubuntu fake RAID howto, How do I differentiate “fake RAID” from real RAID?