It is already known this is not possible mdadm --create /dev/md3 --level 10 --layout=f2 --raid-devices=2 /dev/sdc1 /dev/sdd1 mdadm: /dev/sdc1 appears to be part of a raid array: level=raid10 devices=2 ctime=Sat Dec 24 18:44:29 2016 mdadm: /dev/sdd1 appears to be part of a raid array: level=raid10 devices=2 ctime=Sat Dec 24 18:44:29 2016 Continue creating ar........

Here is the scenario you or a client have a remote machine that was installed as a standard/default minimal Centos 6.x machine on a single disk with LVM for whatever reason. Often many people do not know how to install it to a RAID array so it is common to have this problem and why reinstall if you don't need to? In some cases on a remote system you can't easily reinstall without physical or KVM access. So in this case you add a second physical or disk or already ha........

LSi Megaraid At first it was configured as a RAID 0, then I deleted the Virtual Disk Group. I thought both drives would be shown and detected in Linux as sda and sdb but it actually shows nothing. To make them work you have to hit Ctrl+R before the system boots (when prompted) and create a Virtual Disk Group. In my case I created each one as RAID 0 (with a single drive only) as I just wanted JBOD but there is no such option or default in these Dell Pe........