"Unable to write to destination. Retrying..." error

Comodo Backup running with admin rights in Win 10 x64.
Attempting data backup of local files to a 2nd local hard drive.
Backup starts fine but after about 10 minutes hangs on a file with a message “Unable to write to destination. Retrying…”
It never seems to retry.
Tried volume shadow copy as well as raw read mode.
When it reports the error, CAB becomes unresponsive and won’t abort or close without ending the task. System must be physically rebooted as Windows won’t complete shutdown.
I managed to backup my small C system drive fine but the data drives are hanging CB.
I’ve ran chkdsk on the NTFS filesystems on source and destination and everything is fine. Destination has 3tb free, backup is 1tb.
If I retry after reboot it it will hang on a different file each time so it’s not the files themselves.
Windows 10 is a clean install with only Windows Defender and Windows firewall.