Volume Shadow Copy Service operation failed, error 0x800423f0

I run WS08 on my laptop, mostly so I can run Hyper-V, but also because it seems to provide a better Vista experience than Vista does. Part of what I like about WS08 is Windows Server Backup. WSB has some quirks and hassles (e.g. system state backup is just awful), but for the basic "back up your machine to a hard disk" scenarios, it's easy and fast. I generally back up to my BFS[1] over the network, which takes about 20 minutes for a full 90GB.

I've been building virtual machine images for the workshops I'm doing for Pacific IT Pros next week, and after I got the Active Directory Disaster Recovery VMs all put to bed, I thought it would be a good time to back up the machine. I ran WSB and tried to perform a full backup to the BFS, and I received the error "Volume Shadow Copy Service operation failed. Error 0x800423f0. Backup not started." Ick, I was not expecting that.

A little poking around determined that the error was due to the inability of one of the VSS writers to create a consistent snapshot prior to the backup. I ran VSSADMIN to see if it would provide a clue, with the following results:

Microsoft Windows [Version 6.0.6001]
Copyright (c) 2006 Microsoft Corporation.  All rights reserved.

C:\Windows\system32>vssadmin list writers
vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
(C) Copyright 2001-2005 Microsoft Corp.

Writer name: 'Microsoft Hyper-V VSS Writer'
   Writer Id: {66841cd4-6ded-4f4b-8f17-fd23f8ddc3de}
   Writer Instance Id: {6162e336-7448-4371-93a7-29581512b103}
   State: [8] Failed
   Last error: Inconsistent shadow copy

Writer name: 'System Writer'
   Writer Id: {e8132975-6f93-4464-a53e-1050253ae220}
   Writer Instance Id: {b1c47cec-0e12-4ac6-b84b-9c1d8292a9f3}
   State: [5] Waiting for completion
   Last error: No error

Writer name: 'Registry Writer'
<... more similar stuff...>

The interesting bit is the Hyper-V entry. It apparently was the VSS writer that could not establish a consistent snapshot. Not coincidently, I had just finished up building a bunch of images with Hyper-V.

The cause of the problem: I built my VMs using differencing disks (a type of virtual disk that starts with a read-only base image, and writes all modifications to a separate image). To improve performance, I placed the base image on my laptop's internal drive, and the differencing images on an external USB drive. When I started the backups, I had disconnected the external USB drive. The VSS writer for Hyper-V tried to build a consistent snapshot of the differencing drives, and couldn't because the differencing images were offline. Hence the error.

I plugged the USB drive back in, and fired up the backup with no trouble at all.

Note to WSB product team: A little more effort in the error message department wouldn't hurt.

[1] BFS = big freakin' server

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