SBS2003 Backup to external HDD fails

hi there,

I've been having some trouble with the built in backup in sbs2003 - typically the backup will fail with the following logged-

(in addition, in the event log I have the follow errors- VSS event ID 20 & 8193, ntbackup event ID 8019, sbs backup event ID 5634)

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24/02/2009 23:00
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Date: 24/02/2009
Time: 23:00
User: SYSTEM
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Backup Runner started.
Launching NTBackup: ntbackup.exe backup "@C:\Program Files\Microsoft Windows Small Business Server\Backup\Small Business Backup Script.bks" /d "SBS Backup created on 24/02/2009 at 23:00" /v:yes /r:no /rs:no /hc:off /m normal /j "Small Business Server Backup Job" /l:s /f "E:\SBS Backup\Backup Files\Small Business Server Backup (02).bkf" /UM
NTBACKUP LOG FILE: C:\Documents and Settings\SBS Backup User\Local Settings\Application Data\Microsoft\Windows NT\NTBackup\data\backup10.log
=====================<BEGIN NTBACKUP LOG FILE>=====================
Backup Status
Operation: Backup
Active backup destination: File
Media name: "Small Business Server Backup (02).bkf created 24/02/2009 at 23:00"

Volume shadow copy creation: Attempt 1.
Timeout before function completed

Error returned while creating the volume shadow copy:0xffffffff.

Error returned while creating the volume shadow copy:ffffffff
Aborting Backup.

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The operation did not successfully complete.

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=======================<END NTBACKUP LOG FILE>=====================
NTBackup finished the backup with errors.

For more information about failed backups, see the article on troubleshooting your backup at the following Web page: http://go.microsoft.com/fwlink/?LinkId=18414

Backup ended at 24 February 2009 23:05
Backup Runner finished.
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