Trying to boot from another user's external system disk

This computer has a Touch ID Sensor built into the keyboard ?

Mac computers with the Apple T2 Security Chip - Apple Support (CA)

You are going to have to use the info in below link to change the Security Startup setting

Thereby allowing the computer to boot from a External Drive

About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

How was the external boot SSD created? If it was cloned from your wife's older macOS installation, then it may not have the necessary drivers to work with your newer 2019 laptop. A clean install of macOS 10.15.2+ must have been installed on that external SSD boot drive so it will have the necessary drivers compatible with your 2019 laptop. If a clean install of macOS 10.13 or even 10.15.1 was installed on that SSD....even if later updated/upgraded to 10.15.2....will not have the necessary drivers for a 2019 laptop. A MBP 16" (2019) laptop originally shipped from the factory with 10.15.1 (19B2093) installed which is why I mentioned a clean install of macOS 10.15.2+ must be the starting point of any OS on the bootable SSD.

P. Phillips wrote:

That is helpful IMHO 👍

Though does mention Monterey 😩

Yes, but how was Monterey put onto that external SSD? If it was an upgrade from an older OS only compatible with the 2017 model, then it is unlikely to have the drivers necessary for a 2019 laptop. If macOS 12.x Monterey was a clean install even on a 2017 model, it would have all the drivers available with Monterey so may work with a 2019 laptop. At least this is how it always worked years ago....no idea if it still does today.

If the bootable external SSD had been created from the 2019 laptop, then that external SSD would work on any 2019 or older Mac compatible with that OS.

readroberts wrote:

My wife and I are on a long trip, and brought my laptop ( 2019 16" Macbook Pro, on Ventura 13.4.1) and her external solid state external boot disk (used with her 2017 IMac, on Monterey), thinking that we could boot my laptop off of her external boot disk when she needed a computer with her stuff on it. When I set her external disk as the startup disk and restart, then progress bar seems to hiccup at one point, and then it boots onto my disk. If I bring up the recovery manager by holding down Option-R after rebooting, the menu lets me choose her disk, but then tells me I don't have permission to boot from it. Any way to work around this?

If you can't find success from the suggestions by P. Phillips and HWTech, then a less desirable but possibly workable solution might be to use Migration Assistant to try to migrate ONLY her user account from the external drive over to your laptop.

I have done this before successfully but not recently on Ventura.

No problem. It is a bit confusing how that works and difficult to explain as well. You gave me another chance to clarify, so it should better help the OP and any others seeing this thread. All is good.

That is helpful IMHO 👍

Though does mention Monterey 😩

All sound points you have made.

Sorry if I appeared have under minded you earlier points.

It was not intended

👍