On power up the F10 key will get you into the BIOS which has an American Megatrends Inc copyright message. At the top of the screen it's identified as the Hewlett-Packard Setup Utility. The 'System Information' screen identifies it as BIOS Revision L42 v01.04 dated 08/31/2016. I subsequently upgraded it to L42 v01.06 dated 07/31/2018.
For BIOS version(s) see the BIOS versions page.
The F2 brings up a diagnostic screen.
With an 8GB SanDisk USB pen drive plugged in to one of the front ports I got:
Under Storage->Storage Options
External USB Storage Boot ▶Before Internal Storage Legacy Support Enable
Under Storage->Boot Order it displays a screen listing:
▶UEFI Boot Sources USB Floppy/CD USB Hard Drive Legacy Boot Sources USB Floppy/CD Hard Drive SanDisk USB DISK 3.0 4.0A Network Controller 1 (Realtek PXE B01 D00)
You hit enter to select a particular device and use the up/down arrow keys to position it in the hierarchy.
There is also a 'Security' screen that lets you enable/disable a lot of things. For example you can individually enable/disable the four USB ports. In fact there is a lot you can play with in the BIOS settings.
Note Despite setting everything as described above I found, with BIOS v01.06, the unit always booted from the internal USB drive. The only way I got it to boot from the external USB drive was by removing the internal drive. At the moment I cannot explain this behaviour.
On the board, between the battery and the DVI socket there is a jumper labelled as E49. To reset the BIOS password it is a simple matter of:
When you next power up ant Setup or Power-On passwords should have been cleared.
My t420 came with Thinpro on it. I decided to update it to the latest version. HP provide a Windows application for downloading and updating firmware. It's HP ThinUpdate which at the time of writing is sitting at version 2.6.6.
As noted above I updated the BIOS for the t420 to version L42 00.01.06. Out of interest I decided to use the 'tool-less' BIOS-based update path for this. It took me a while to sort this out as it is poorly described and not as straightforward as you'd think. I assumed it would be as simple as putting a binary file on a USB pen drive and going from there, but no it wasn't. It involved running some scripts within ThinPro to move the BIOS file to a given 'known' location and then rebooting.
Unfortunately this was some time ago (as I write this) and I didn't make comprehensive notes at the time. This section will get updated if and when I have another example to work with.