Hit F2 at power-on to get into the BIOS. This describes itself as Aptio Setup Utility and is an AMI BIOS and reported 'Core Version 184.108.40.206'. Selecting System Information took me to another screen where it gave the BIOS version as R1.16.0, built 01/28/2015.
Moving on to the Advanced tab which included the intriguing entry Auto BIOS Update. If you enable this option the S520 goes on-line at boot time and checks the the specified server (webdownloads.ts.fujitsu.com) to see if there is an updated BIOS available. Manually checking the Fujitsu support site I could see there was V220.127.116.11 - R1.18.0 available. When I tried the automatic process it did go on-line but, after maybe 10 seconds, reported "Couldn't get firmware information from update server"
In the boot priority list there are eight entries. When exploring these I had a SanDisk Cruzer pen drive plugged into one of the USB ports. Each entry can be set to one of:
P0: 4GB NANDrive Realtek PXE B01 D00 SanDisk UEFI: SanDisk UEFI: SanDisk IP4 IP4 Realtek PCIe GBE Family Controller IP4 IP4 Realtek PCIe GBE Family Controller Diagnostic Program Disabled
My test SanDisk pen drive had two partitions on it which I guess is why there are two appearances of 'UEFI: SanDisk' in the above table.
For a one-off selection you can hit F12 at power-on to get straight into the boot screen menu without having to mess about in the BIOS.
The BIOS update on the Fujitsu support site is actually headlined "BIOS Update - Windows for D3314-A1x" and comes bundled with 'Desk Flash Instant' that runs under Windows. The download is a self-extracting exe that immediately launches into the BIOS reprogramming if you run it. If you're interested in just recovering the files then use 7Zip (or similar) to unpack the individual files.
Due to way it is named and packaged I assume this is something to fix a Windows issue and the update is deemed unnecessary if you are running eLux.
I haven't looked into how you update the BIOS if you're not running Windows.
My S520 originally ran eLux RL but the flash had been wiped clean by the supplier. The variant fitted with 16GB of flash maybe running Windows Embedded Standard.
Any comments? email me. Added August 2019