I'm basing this off my R410, so it may differ a bit for an R710.
When you boot the server, hit F2 to enter System Setup.
Once in System Setup, navigate to "SATA Settings".
Set the SATA Controller mode to "AHCI Mode".
If your hard drive backplane is configured to use the on-motherboard controller, any modern Windows OS should be able to see the hard drives in this mode. I do not recommend RAID with this controller as performance is going to be absolutely terrible.
If this does not work, you likely have a secondary controller installed like a PERC6/i.
To check if a PERC6/i can see your drives, you should see something like "DELL SAS 6 blah blah" come up during the boot process and will need to hit CTRL+C to enter it's setup.
Once in the config utility, you can hit enter to navigate to the SAS6IR device properties.
From there, go into SAS Topology. Then hit enter over Direct Attached Devices.
You should see a list of the hard drives attached if the controller sees them.
If the first controller didn't show any devices during post, and the second method lists no devices, something isn't plugged in somewhere.
If the second method lists devices but Windows doesn't see them, try searching for drivers to load during Windows installation that would allow the OS to see the drives.
Unfortunately these servers can come in so many configurations that all of the above may not apply to you.