Your picture shows RAID-1, not RAID-0. The set is "degraded" because there's no mirror partition. There's just one partition in the RAID set. The system is simply telling you that there's no mirroring going on.
That said, you also appear to be using two external hard disks for this purpose, apparently the idea being that they would be RAID-1 mirrors of each other. The problem is that both disks are USB disks. USB shares it's bandwidth across all devices and writes to devices will occur sequentially. By making a RAID set across the USB disks, you have the effective bandwidth to each disk and increase the latency. The practical upshot being that if the RAID is properly configured, the performance will be < 50% that of either drive individually (and the CPU usage to handle that much USB traffic and the mirroring will be pretty high too).
A better solution for RAID-1 is to purchase a dedicated enclosure. Such dual-drive RAID-1 enclosures don't cost too much, and they place the RAID-1 controller directly in the enclosure. From the standpoint of the OS, it appears as a single disk, and you get 100% the performance of the disks with minimal CPU overhead. A firewire enclosure is even better (performance-wise), if you can find a decent one.
