Running my Lat 7300 on BIOS 1.6.1. Can confirm, switching off SMM Security Mitigation allowed me to install the Thunderbolt 3 firmware update. No need to downgrade the BIOS for me. Thanks, @jbeckers for finding this out.
Thanks for the hint. Mine is 7400 2-in-1 and newest BIOS is 1.6.0. Don’t know if it’s the same issue and fix. Is this SMM setting in BIOS? Will check it out in a few days when I get reach to the laptop.
Someone sent me a similar tip about disabling SMM Security Mitigation on a Latitude 5400 (and 5500) when trying to install the latest TPM firmware they made available recently. On the latest BIOS version, it would lock Windows solid the moment you ran it.
Dell support had me trying all kinds of things but never once had me check that BIOS option, so they must be unaware of the impact of this across different models.
One very annoying thing I noticed: I can't disable that option using CCTK. If it's already disabled, I can enable it, but going from enabled -> disabled gives an error message. So if you're trying to automate anything, forget about it.
Link to the other post, in case it helps people with the 5400/5500 model find this info easier: