Hi [NAME],   H710 supports OCE (Online Capacity Expansion), hence you can add 1 or more drives to your existing RAID5. You can refer to this article [URL] on how to add the drive into your existing RAID. Since you mentioned you have already done data back up, I need not advice you to do one full data back up. You can do to configure via both Ctrl+R or OMSA.      Ref: Page 10 OCE [URL]
I have a Dell PowerEdge T320 ( ) that is running Raid 5 with three Dell Savvio 10K.6 300GB HDDs using the Dell PERC H710 SCSI controller and BIOS Configuration Utility 4.04-003. I have two extra Savvio 10K.6 300GB HDDs, and want to add one of them to the current 3 disk set for a total of 4 disks. I was told that adding an additional drive could or should increase performance from the drives by as much as 20% to 30%.  So, I just want to confirm the correct method to accomplish this? I have already backed up the system. Should I use the CTRL+R during boot up or run the OMSA (OpenManage Server Administrator) from Windows to accomplish this? What are the steps for making the disk become a member to the current disk set?   Thank you Mike  
Adding a 4th Disk to a RAID 5 with 3 Disks