Error writing metadata

Just to concur on a couple of things: Bridge is a viewer with lots of capabilities. But when it's looking at a file(s), and you assign various metadata to that file(s), when you leave that file and go onto the next, the data remains with that file(s). If Bridge crashed, it's not going to go back and delete all of the things in the file(s) that it is no longer working with.

This is no different than if you opened a Word file, did something, left that file, went onto the next and then Word crashes. The contents of that first file are not affected. The only difference in my analogy is that you have to save the Word file but when you add metadata to a file, when you close that window the contents are self-saved.

If all of your changes were lost, something else happened, it wasn't Bridge that lost those changes. It can't have been.

Now, that doesn't mean something didn't happen, obviously something did. There are several things that I'd like you to try and maybe we can boil this down and find the issues. My guess is that something happened on your Mac that is causing the error message that you're getting AND caused the loss of metadata.

One thought here before I provide you with a buch of maintenance tasks: you said you have some 20 K images on your main hard drive. Really? How big is your hard drive? And much more important, how much free space do you have left on your computer? Generally, you should always have at least 1/3 - 1/4 hard drive space left on your computer at all times so it can do what it needs to do. I wonder if this might be some (or all) of your problem. Please check this now and if your hard drive is almost full, get yourself two external drives and start moving your images onto one of the external drives (the 2nd one is to back up the first drive).

Otherwise, here are some self-maintenance things that you can do that will cost nothing but a little time.

1) Go Download Catalina Cache Cleaner. It's free to download and you have a full use trial period. In the end, if you don't like it, toss it but it has helped me a number of times. If you buy it the charge is nominal. It does a variety of things but the two you should try is the general maintenance operations, click on ALL and then click on Run. (These are things that are supposed to be self-run by the Mac OS while the computer is asleep but this just guarantees that they are done.) Then click on the Cache tab, select the Middle option and run in on all Caches. (Note: it will seem like nothing has happened  but wait a few minutes and then you'll have to restart your computer. Just be patient)

If that does not clear things up, then...

2) Go into Safe Mode. Restart your computer and when the screen goes black, press the Shift key until the progress bar is displaying for a minute or two. This startup will take a lot longer than you think it should and if it's much much longer, that means that the Mac is doing a lot of repair on your system. Your Mac will run ridiculously slow after you start up and if you want to see if Bridge is crashing or not you can but I can't say it will make much of a difference. 

If that does not clear things up, then...

3) Go into Repair Mode: Restart your computer and when the screen goes black press the Option + Command + r keys. When you see a glob spinning, you can let go. This will take a long time, 5-20 minutes depending on your Internet speeds. During this you may or may not be asked to sign into your wifi (if you're on wifi). [One option to get around this delay is if you have a partition with an operating system on your computer or a separate hard drive to boot up off of.] Now go into Disk First Aid and see this dropdown menu in upper left corner of the DFA and select Show All Devices:

(If you do not see this, restart into a normal startup and then repeat #3.) Once selected, you should see your hard drive displaying something like this. (It will be different because I took these screenshots running DFA while I'm writing this.)

Now, run this from top down. That is select a top offering, and run the process. If you see any errors, run it again. Then, once clean do the next one, etc. The last one is "(your hard drive)" - Data." Once that one is running clean, restart.

Once these are done, then restart Bridge with your fingers on the Shift-Command-Option keys. This will bring up a window asking if you want to trash your Bridge's preferences (or something like that). Click yes. See if that solves the error messages.

Lastly, good luck!!

Once you assign metadata, a crash doesn't magically make it go away. As for the errors, you could have file corruption or (more likely) permissions errors. Look through posts on here, there are a number with info about fixing permissions.