You can do this either way, but I would suggest waiting to configure your RAID 10 until after the OS is installed on the RAID 1. Before: If you configure both RAID 1 and RAID 10 before you install the OS, this is fine, but you will need to watch what you are doing.  If installing 2008 R2, you will come to a point where Windows asks you where you want to install, and it will show you two "disks" - one is your RAID 1 and one is your RAID 10.  As long as you select the right one, there is no problem. After: If you configure only the RAID 1 to begin with, then when you install the OS, there will only be one disk option to install to.  Once Windows is installed, then you can install OpenManage Server Administrator and configure the RAID 10.
General question - we are getting a new T620 with a PERC 710P card with the intention of setting up RAID 1 for the OC on C:\ and RAID10 on D:\....I have been reading the PERC manual and need to know how to go about this......would I configure the RAID1 on the first array and then install the OS, then go back and setup the RAID10?  What I am asking is how do I ensure the RAID1 is the C:\ drive and the RAID10 is the D:\ drive?
T620 Raid Setups