Cannot get Windows Backup to complete successfully! Error 0x81000037

Back in 2015 I set up a USB 3.0 external HDD for backups, and used Windows Backup for awhile to make weekly backups of the system. It worked for a short while, then the backups started failing, and I never got around to fixing it. I went over a year with no real backup in place.

Now I'm trying to get the backups going again, and so far have been unsuccessful at every turn.

I have removed all previous contents from the USB drive and reformatted the drive several times today. Every time I run Windows Backup and set it up to run at 1 AM every day, it starts the backup immediately I guess to create the initial backup, and after 1-2 hours it errors out with error 0x81000037, and the message: "The backup was not successful. The error is: Windows Backup failed while trying to read from the shadow copy on one of the volumes being backed up. Please check in the event logs for any relevant errors. (0x81000037)."

I did some reading and thought the problem was caused by choosing to backup the root C: drive plus several of the subfolders on C:. I tried NOT checking the root C:, and instead only selected a few of the subfolders. That didn't work.

I tried not backup up C: at all, and instead only backing up my user profile folder and the entire D: data HDD, plus checked the option to make a system image of C:. Still no good.

Any other suggestions what to try? I had a catastrophic HDD failure in 2009 with no backup in place, and I'd rather not go through that again.

Yeah tried that, there were no "Mounted Volumes" found at any of the listed junction points.

I made sure VSC and the Backup services were all running, turned off Windows Defender and unplugged my network cable, and finally the backup completed after running about 8 hours.

Now I seem to be having an issue where Windows is making the tone when removing a device, over and over at random times, and it's popping up a message saying I need to format Drive F (the USB backup drive) before I can use it, with choices to Format or Cancel. I keep hitting Cancel, and checking Windows Explorer, and the F drive comes up just fine. Not sure why it keeps giving me the message that it needs to be formatted.

Yeah I've been thinking of doing that. I built the system almost 4 years ago May 2013, and have not reloaded or reformatted anything since then.