Can you try to use a Support Live Image to access OMSA. You can check below link for details [URL] 
I have a PowerEdge R710 with a 6 x 3.5" front backplane connected to the internal PERC H700. I have ProxMox installed on a 4TB physical hard drive that's set up as a one-disk RAID 0. I have a 1TB physical hard drive that's also set up as a one-disk RAID 0. I recently added another 1TB physical hard drive that's not assigned to any virtual disks. I'd like to add this disk to my second virtual disk, so as to create a two-disk RAID 1 mirror. Everything I see on the internet tells me this is possible. The BIOS has been updated to 6.6.0. The Lifecycle controller is running version 1.7.5.4. The iDRAC 6 Enterprise is version 2.92. I know that I can obviously destroy the 1TB one-disk RAID 0 and then take that drive along with the other 1TB to make the two-disk RAID 1 mirror, but I want to do it without losing data. At some point, I'd like to do it with my 4TB drive (with a second 4TB drive) and since that has the OS installed on it I'd rather not have to start over. How can I convert my one-disk RAID 0 to a two-disk RAID 1? p.s. if all else fails I think I can just boot to a rescue disk, image the virtual drive to another storage location, destroy the one-disk RAID 0, create a two-disk RAID 1 mirror, then restore the image back to the newly created virtual drive. But I'd like to figure out how to do this ... natively.
PowerEdge R710 with PERC H700 - Convert from one-disk RAID 0 to two-disk RAID 1