You can also just use Repository Manager and have it effectively build the SUU for your R720XD. It downloads the individual patches and then builds them into an ISO (or share if desired). This may be more successful as it doesn't have to download 1 huge file, but just gets multiple smaller files.
Been trying all afternoon to download the new SUU (16.04), but every time it either ends early with an incorrect file size or barely gets started before failing.  I am also trying the lifecycle controller updates and it's not going anywhere either.  Is something bad happening at Dell?
Unable to download anything from Dell's support/drivers pages