vol hash mismatch

I think my problem is solved. It only took a month. I tried and tried and could not get Monterey 12.3.1 to download without it failing. I contacted support using virtual chat. It was suggested that I use safe mode to start my iMac. I did this. It must have cleaned up something because the message Volume Hash Mismatch went away. The mail program did a rebuild. I went to do the update again in software update. The highlighted button said system restart. Pressed that and the system restarted. After the restart I checked software update....Your Mac is up to date macOS Monterey 12.3.1. Success.

The external hard drive I was using for Time machine backups stopped working. It was not able to write to it. This happened at the same time as the Vol hash mismatch message. I had tried to update to Mac OS Monterey 12.3. That failed. I turned off my computer overnight. I started up the computer and no error message. I replaced the external hard drive today. It is currently doing a time machine backup. It will take a while. I have not seen the error message. Computer seems stable. It was randomly restarting.