This could be some driver issue and operating system not able to detect the controller and virtual disks. As this controller is not supported on PowerEdge R820 we can not guarantee that it will work.  Can you check in device manager whether the driver is installed for the controller and it is showing properly. If you have access to iDRAC, You can check in iDRAC whether the controller is inventoried or not
I had an H840 card in a R820 connected to MD1200 enclosure. I know this is not an officially supported config, but everything has been working fine (for years). Today, I moved the H840 from the R820 to a R620. BIOS detects the card fine. If I enter into the bios, I can see all the connected drives, virtual disks etc fine. But, once I get into windows, I do not see the disks in Windows DiskMgmt.msc nor do I see them in OM Storage. But I see the card in OM Slots.  I updated my OM (to v10.3) but the behaviour stays the same.  How do I get this card to show in storage & windows? [IMAGE]  
PERC shows in BIOS, not OM Storage nor Windows