Ventura, external USB drives and refusing to mount, unmount or do First Aid

I have 2 x WD Elements 4TB external drives with USB3 connectors. 1 is a current backup and archive, and the other is a spare. Both are formatted for ExFat so I can use them on a Windows machine.

Before I upgraded to Ventura on my 2017 MacBook Pro, I was able to use these drives without issue, shuffling files on and off backup / archive as needed.

I upgraded to Ventura a few days ago, after which I've experienced a nightmare. I also moved to iCloud from Google Photos, which meant transferring 41,000 images across 350GB. Part of this was to refresh the backup of my photos, using the new Apple Photos library as the master.

Long story short, neither drive mounts or dismounts properly. Either it will:

• mount, then refuse to unmount, or

• it won't mount until I've run First Aid in Disk Utilities. In this case, First Aid fails and mounts the drive anyway. It will then refuse to unmount.

Sometimes MacOS reports it can't unmount a volume due to an application still using it (even though the application hasn't opened a file on the drive, and isn't open on the Mac). Occasionally it reports Finder is still using it. More often than not it either sits there and does nothing but go quiet. I've left the disk in place for up to 3 hours to no avail.

The backup drive also refuses to empty its trash can. Even running through individual files and clicking "Delete Immediately" has produced errors. The spare has nothing in its trash.

So far I have tried:

1. Using different ports and cables (obvious, but worth a shot).
2. Starting in Safe mode. The disks mounted, but refused to run First Aid and then refused to mount.
3. Attempting to run First Aid on the same disk multiple times. This did nothing but prove Einstein right.
4. Reformatting the spare to MacOS Journal. This worked when there was nothing on the disk. Once I put a few files on it, it returned to refusing to mount / unmount.
5. Reformatting the spare to exFAT. Again, this worked while there was nothing on the disk.
6. Running ChkDsk /f via Windows. This found and cleared an issue on the backup drive. Inserting the checked drive into MacOS saw it mount correctly, but refused to unmount. Running ChkDsk /f on Windows again found and cleared issues. 
7. Downloading files from the backup directly to the spare. This failed due to reported corruption in individual files.
8. Downloading files from the backup directly to my Mac, then to spare. Files were passed without error to my Mac, then reported corruption errors when uploaded to the spare. However, if I then copied the individually affected file there was no error. Nor was there an error when I passed it back to the backup.

If I don't use my Mac and pass everything through my Windows Laptop there doesn't appear to be a problem. Chkdsk reports no issues after multiple read/writes.

Given I successfully moved 160gb to backup the day before Ventura, and entered **** the day after, my working assumption is something in MacOS Ventura has changed.

Any thoughts?