Having a bootable driver update that contains everything your Dell R420 server needs is the way to go.
(Updated = We are now able to allow the full download of the ISO file. So there is only ONE file to download now.)
File Download
R420_bootable.iso
Size: 3.6GB
R420_bootable.iso« Download
[Create Bootable USB Drive]
Next, use [Rufus«] to create a bootable USB drive.
Set the following options.
  1. [Device]: USB Drive
  2. [Boot Selection]: R420_bootable.iso
  3. Load the ISO into Rofus and Set the following
  4. [Partition scheme]: MBR
  5. [Target System]: BIOS or UEFI
  6. [File System]: Fat32
  7. [Cluster Size]: 4096 Bytes
(Information may vary for your server. This is the basis for the R-Series Boot Options.)
[Dell R420 Server - Boot from USB]
Next,
Error
Issues you may encounter and what to do
(This seems to be an issue with all Dell R Series servers that have the iDrac Express card installed.
The Question mark indicates the Version Number.)
#1: Device: iDrac
Application: iDrac
Failed to update to standby image.

This issue will happen if the iDrac Express card is no longer functioning.
Replacing the iDrac Express card is required.
Dell Drivers Download Page
R-420 Drivers Page«
The man behind all the ISOs
Allen Sampsell«