Having a bootable driver update that contains everything your Dell R520 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
R520.iso
Size: 3.5GB
R520.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]: R520_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
[Dell R520 Server - Boot from USB]
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-520 Drivers Page«
The man behind all the ISOs
Allen Sampsell«