Best way to get the available disk space?

I'm still relatively new to .NET and iOS, so sorry if this is a dumb question I'm trying to determine how much free storage is available on the device, and the simplest way seems to be using DriveInfo.GetDrives()[2].AvailableFreeSpace. On my iPad 3, GetDrives() returns a total of 4 drives, and all but the 3rd have 0 space. Is the 3rd drive always going to be the system drive in iOS or is there a better way to determine free space?