I'm not entirely certain how this drive works and interacts with the file system even after skimming the user manual. I'm not sure if this is your exact drive or not:
https://downloads.wdc.com/sdp/docs/um-armorlock-ios.pdf
However, it appears you can manage this drive from either an iPhone or the computer. I would suggest trying to manage this drive using your iPhone since you may have third party software installed on your computer which is interfering with the normal operation of macOS and the SanDisk utility on the laptop. If your iPhone can unlock the drive, but you are having trouble accessing items on the drive from the laptop, then try booting the laptop into Safe Mode to see if that makes a difference after using your iPhone to unlock the drive again. Usual culprits are anti-virus apps, cleaning/optimizer apps, and third party security software....none of which are needed on a Mac since they interfere with the normal operation of macOS. If you have any of these types of apps installed, then uninstall them by following the developer's instructions. This could explain why the firmware update did not work.
From the user manual, only the person designated as a "Manager" can configure the drive. The Manager can configure other people to be able to unlock this drive. This drive is very complex with very little technical details on how it all works. If you are unable to unlock the drive, then you may need to reset it to factory defaults which would destroy all of the existing data on the drive. The user manual has instructions for this process (page 22 of the manual I linked in this post).
I highly recommend you experiment with this drive for a while until you understand how everything works with it and confirms it is working properly. If you must reset the drive to factory defaults, then you may want to prevent the drive from locking just so you can confirm the basic drive is working fine with your laptop before you start enabling the more advanced & complex security features.
Also, it may be a good idea to use your iPhone to manage this drive at first to make sure it works Ok before you start using the manufacturer's proprietary third party software on your laptop since your other third party apps could interfere with it and you may need to also worry about the macOS Privacy & Security settings as well which can possibly add more complications. You may discover the drive works fine when managed by the iPhone, but has issues when you use the laptop to manage the drive. This would at least narrow down where the problem is at (some sort of software or configuration issue with the laptop). If you have issues even when you are only managing the drive using the iPhone to unlock it, then you either have an issue with the SanDisk drive and/or the SanDisk software which only SanDisk & WD can assist you.
FYI, you can use your iPhone to update the firmware of this external drive....may be a good option depending on the third party software installed on your laptop and the laptop's own privacy & security settings which could affect a firmware update.
If you continue to have issues, you may need to contact SanDisk (WD) tech support for assistance in using this drive with a Mac. They should be able to help you to figure things out and answer your questions about how to properly use this drive.
There are much simpler Self Encrypting SSDs (SEDs) out there than this one where all you need is to use the drive manufacturer's proprietary third party app to unlock the SSD. These simpler SEDs only require a password to unlock the SSD's built-in hardware encryption, but they don't have all the extra complexity of managing users like this Armor Lock SSD does. The more complex something is, the more likely something will or can go wrong. Do you need all of those advanced features of this Armor Lock SSD? Anytime any drive manufacturer's proprietary third party software is involved to manage a drive, things can go wrong. I can also appreciate wanting or needing to use a SED as well.
