Windows Server 2008 Installation on Dell PowerEdge R720 – Troubleshooting Guide

See Perc H710 User Guide, p 18 :

Reconfiguration Of Virtual Disks

An online virtual disk can be reconfigured in ways that expands its capacity and/or change its RAID level.

NOTE: Spanned virtual disks such as RAID 10, 50, and 60 cannot be reconfigured.

As I understand it, Raid 10 VD cannot be expanded.

I couldn't find a way to make the server itself tell the Windows installer how to see the disk, but by talking to @joeqwerty in the comments I ended up following this course, which worked:

1. Download the latest drivers from Dell: I did that from my product's support page, going on "Drivers & Downloads", category "Driver for OS Deployment". There was one file available, "Dell OS Driver Pack, Version 14.11.02, A00" which was the same version already in the Lifecycle Controller. The direct link was: http://downloads.dell.com/FOLDER02672561M/1/Drivers-for-OS-Deployment_Application_GCNFF_WN64_14.11.02_A00.EXE
2. unzip the exe. Inside it, there's a file "mas021.zip" inside the folder "payload". I unzipped it too, and copied to an USB.
3. During the installation, when it reaches the disk part, I selected to "Load Driver", "Browse", and went to the folder "48V2R", where windows found the driver for "PERC H310 Adapter".
4. After that, I was able to partition the disk and proceed with the installation. Still in the disk screen, the installer still had a warning saying that "Windows cannot be installed", and as a reason it says the hardware might not be able to boot on this disk. However, the "next" button is enabled and I was able to install windows. Also, after installing, I was able to reboot ok, so that warning was wrong.

On the good side, I was able to install windows without any further errors. On the bad side, I don't have network: it doesn't even display the network cards. That means I should have drivers for that too, and who knows which other drivers are missing. Probably if I was able to install windows using the Lifecycle Controller properly in the way it was intended, I would now have all the drivers I need, but I couldn't find out how to do it.

Thanks @joeqwerty for suggesting that I update the OS drivers, which led me to this partial solution.

P.S.: after I did that, I realized that I was using a Win2008R2 without SP1. I repeated the installation with a new disk with SP1, but the only difference was that I didn't get the message saying that the hardware might not be able to boot on this disk. All the other problems were the same, and the same workaround went fine.