NTBackup - 'The network disk drive has stopped responding'

[age]

Hi, I am getting the following error when running backups using NTBackup on a Windows 2003 server (all fully patched).
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Backup Status
Operation: Backup
Active backup destination: DLT
Media name: "Media created 08/08/2006 at 16:54"

Volume shadow copy creation: Attempt 1.
Backup (via shadow copy) of "C: "
Backup set #1 on media #1
Backup description: "Set created 08/08/2006 at 16:54"
Media name: "Media created 08/08/2006 at 16:54"

Backup Type: Normal

Backup started on 08/08/2006 at 16:56.
Backup completed on 08/08/2006 at 21:00.
Directories: 10149
Files: 153808
Bytes: 45,243,800,330
Time: 4 hours, 3 minutes, and 17 seconds
Backup of "SVR-01\Microsoft Information Store\First Storage Group"
Backup set #2 on media #1
Backup description: "Set created 08/08/2006 at 16:54"
Media name: "Media created 08/08/2006 at 16:54"

Backup Type: Normal

Backup started on 08/08/2006 at 21:00.
The network disk drive has stopped responding. Backup set aborted.
Backup completed on 08/08/2006 at 21:00.
Directories: 2
Files: 2
Bytes: 10,485,956
Time: 10 seconds

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So it is failing to back up the Information Store. Any ideas?

The script that runs from a scheduled task is:

NTBACKUP.EXE BACKUP "@C:\Backup\DailyBackup.bks" /j "Full System Backup" /v:yes /SNAP:OFF /hcn /m normal /l:s /r:no /rs:no /um /p "DLT"

Thanks for your help!

Adrian.