I have used the "Windows 7 File Recovery" tool under Windows 8 to create system image backups to an external USB hard drive. I built a new Windows 8.1 machine, and I want to create my first system image backup of that machine to the same USB hard drive. The "Windows 7 File Recovery" tool is gone in Windows 8.1, but wbAdmin is alive and well:

wbAdmin start backup -backupTarget:\\?\Volume{2a2b...994f} -allCritical -quiet

fails with this text displayed:

wbadmin 1.0 - Backup command-line tool
(C) Copyright 2013 Microsoft Corporation. All rights reserved.

Retrieving volume information...
This will back up (EFI System Partition),(C:),Recovery (300.00 MB) to \?\Volume {2a2b1255-3a86-11e3-be86-b8ca3a83994f}.
The backup operation to F: is starting.
Creating a shadow copy of the volumes specified for backup...
Summary of the backup operation:

The backup operation stopped before completing.
The backup operation stopped before completing.
Detailed error: ERROR - A Volume Shadow Copy Service operation error has occurred:
(0x8004231f) Insufficient storage available to create either the shadow copy storage file or other shadow copy data.

The EFI System Partition is 100 MB
The Recovery Partition is 300 MB
The C partition is 1.72 TB, NTFS, 218 GB used, 1.51 TB free
The destination drive is 1.81 TB, NTFS, 678 GB used, 1.15 TB free

I've fiddled with vssadmin resize shadowstorage, with no change in the error. vssadmin list shadowstorage displays:

Shadow Copy Storage association
For volume: (C:)\?\Volume{37a0...263}\
Shadow Copy Storage volume: (C:)\?\Volume{37a0...263}\
Used Shadow Copy Storage space: 2.39 GB (0%)
Allocated Shadow Copy Storage space: 2.81 GB (0%)
Maximum Shadow Copy Storage space: 531 GB (30%)

Shadow Copy Storage association For volume: (F:)\?\Volume{2a2...94f}\
Shadow Copy Storage volume: (F:)\?\Volume{2a2...94f}\
Used Shadow Copy Storage space: 334 GB (17%)
Allocated Shadow Copy Storage space: 337 GB (18%)
Maximum Shadow Copy Storage space: UNBOUNDED (922154758%)

(Yeah, the "percent calculation" for UNBOUNDED is seriously bogus.)

I've run SFC /verifyonly and it seems happy. I've verified that the new `Volume Shadow Copy" service starts when I start the backup operation.

Any suggestions?