  No, the only thing I can think of is that you are selecting one of the backplane configurations that have the +2 HDD expansions slots. If you select the 3.5" backplane with the +2 slots then you can't use the same HDD's across both arrays. The +2 slots are 2.5" only. If that is not that case then I'll need to know all of the configuration options you are selecting. I have not found a way to get the exact error you are reporting on the R720XD. I get this error when I select 5 3.5" SAS 15Ks and have the +2 HDD bay: Flex Bay (Internal cage) Hard Drive Configuration require Flex Bay 2.5” SAS or SATA HDD or SSD. The Raid 1 set needs exactly two hard with same size, speed and capacity. Please update your selection from the following: Hard Drive module.
Good day, This may not be the right place to ask this but, when I was going through the build options for a R720xd server I got to the HD selection and noticed that since I chose a RAID 5+1 I had to choose at least 2 SATA drives and then SAS drives for the RAID 5. I thought it wasn't a good idea to mix SAS and SATA? Why does the online component options tell me there is an error when I want to choose all 15K RPM SAS drives?   Thanks for any assistance provided.  
RAID 5+1