Windows 10 Backup Failed: Error code 0x81000019

The error message says,

A shadow copy could not be created. Please check "VSS" and "SPP" application event logs for more information.

Details: the writer experienced a non-transient error. If the backup process is retried, the error is likely to reoccur.

I have written a system image to my external drive once but this process has never completed.

I'm backing up to an 8TB (Windows says 7.28 TB) WD MyBook that currently has 5.98 TB free. (The only thing on it is the system image I wrote to it as soon as I bought it a week ago.) In "Manage Windows Backup disk space," it reports the following:

Data file backup: 0 bytes
System image: 1.25 TB
Other Files: 50.32 GB
Free space: 5.98 TB
Total size: 7.28 TB

In Event Viewer, there are two seemingly related errors.

Windows Backup
Level: Error
Event ID: 4100
Backup did not complete successfully because a shadow copy could not be created. Free up disk space on the drive that you are backing up by deleting unnecessary files and then try again.
EventData:
A shadow copy could not be created. Please check "VSS" and "SPP" application event logs for more information. (0x81000019) 19000081980500004409000060090000420ED1665C2BEE174B64529CB14610EA71000000

SPP
Level: Error
Event ID: 16387
Writer ASR Writer experienced some error during snapshot creation. More info: System partition (partition marked "active") is hidden or contains an unrecognized file system. Backup does not support this configuration. (0x80042416).
EventData:
ASR Writer
System partition (partition marked "active") is hidden or contains an unrecognized file system. Backup does not support this configuration. (0x80042416)
The writer experienced a non-transient error. If the backup process is retried, the error is likely to reoccur. (0x800423F4)
System partition (partition marked "active") is hidden or contains an unrecognized file system. Backup does not support this configuration. (0x80042416)
01000000C6120000A71200000000000042BEB7C511CAC619E59C92030000000000000000

System details:
OS: Windows 10 Home 64-bit (Version 10.0.18363 Build 18363)
CPU: Intel Core i5 @ 3.20GHz
RAM: 16.0GB
Motherboard: Gigabyte Technology Co. Ltd. H97-D3H-CF (SOCKET 0)
Graphics:
ROG PG248Q (1920x1080@144Hz)
HP 2159 (1920x1080@60Hz)
Intel HD Graphics 4600 (Gigabyte)
XenSource Citrix Indirect Display Adapter
2047MB NVIDIA GeForce GTX 960 (EVGA)
ForceWare version: 456.71
SLI Disabled
Storage:
Samsung 860 EVO 500GB (SATA SSD)
Toshiba HDWE140XZSTA 4TB (SATA HDD)
Samsung SSD 840 EVO 250GB (SATA SSD)
Western Digital WD My Book 25EE 8TB (USB)
Audio: Realtek High Definition Audio