Hard drive icon flickers from custom icon to folder icon

That's for responding. The backup drive is used by TimeMachine. It's a 4 TB hard drive with only 1 TB used, leaving 3 TB free. The icon is a custom icon that I manually installed.

I have tried to see some pattern in the way it flickers, but I can't figure it out. Once it flickered three times in 20 seconds, and sometimes it will go for an hour without any flickers. There doesn't seem to be any relationship between what I'm doing, or what application I'm using, and the flickering. I have iStat menus installed and I have seen no relationship between the flickering an CPU activity or memory load.

Yes, this is truly weird.

I just did some experiments and I have zeroed in on the exact cause of the flickering. I mounted a thumb drive with a custom icon that I manually installed, then began transferring new files to it and deleting those files. Sure enough, the icon flickered between "folder" and "custom" several times during this process. Similarly, I established that the backup drive icon flickers when Time Machine is writing to it.

This does not appear to be an immediate reaction to changes to the drive; my impression is that there are two threads at work. The first thread is the one writing to the drive. The second threat appears to be some sort of Finder updating thread that rewrites the icon when it discovers that the contents of the drive have changed.

Yep, your first suggestion boxed it in a bit more. I created a new folder on the desktop, gave it a custom icon, and then transferred a file into it. Sure enough, a few seconds later the custom icon was replaced by a folder icon, and half a second later that folder icon was replaced by the custom icon. This happened on the desktop, which is stored on the internal drive, so the problem is not limited to external drives.

I have already confirmed that the problem happens with thumb drives, too.

I checked the page to which you linked showing the procedure for adding a custom icon. I was much surprised to learn that the old method I have used for decades has been replaced by a much simpler method. In the old days, we had to convert an image to ICN format to use it as a custom icon, but now we can just paste an image in and Monterey handles everything. Nice! But alas, the problem remains even with a folder made this way: I transfer a file into it, and a second later it flickers from custom icon to standard icon and back to custom icon.

It now appears that the problem might be due to some application I installed that plays games with the OS. The only two I can think of that might do so are Intego, the virus protection software, and iStat Menus -- not particularly suspicious apps. I have installed no weirdo apps or modified the system in any screwball ways. Hmmm... the plot thickens.

OK, I rebooted in safe mode and experienced the same problem. It flickers when I transfer a file to a folder on the desktop (internal hard drive), the backup drive (external hard drive). I could not, however, get any thumb drives to mount -- rather odd but after I returned to standard mode, I could mount thumb drives.

This plot is definitely getting too thick! 🙃

Thanks for the help. I don't think it's worth any further effort; I don't want to tie down Apple Support on a futile mission. It was worth a try. If a lot of people report that they're having the same problem, I'll try Apple Support and post the solution here.