One interesting things about NTBackup in XP and Windows Server/SBS 2003 is that is has two operating modes: with shadow copy and without shadow copy. First, NTBackup will try to create shadow copies of the involved volumes and try to copy stuff from these special devices to the tape. If a shadow copy creation fails, NTBackup will fall-back on the non-shadow copy method, and it will try to copy these files directly from the live volume devices to the tape.  

Just a quick reminder on what are shadow copies: these are static images of volumes at a certain moment in time. Some sort of a "snapshot" of a volume - imagine your volume contents being frozen in time. Shadow copies are volume devices on their own, with the interesting property that the creation/deletion of these shadow copies is pretty quick (a few minutes even for Terabyte-size volumes). With shadow copies you have certain advantages, the backup process won't  skip open files as in the second case. Also, if you do your backup from a live volume, you might backup inconsistent data since the contents of these volumes is changing in the middle of the backup.

Anyway what we need to remember for now is that both operating modes of NTBackup (with or without shadow copies) should work.

Usually things will work pretty nicely and the backup will complete without errors or warnings. Sometimes, however, NTBackup fails to non-shadow copy mode. There might be several causes, and I enumerated a few of them below:

1) When you are performing a system backup, make sure that all SQL databases are in the "Simple" recovery model. If you have a SQL database with a different recovery model (for example full), NTBackup might panic and fall back to non-shadow mode backup. Again, remember, this system backup should be good enough even in non-shadow mode. To test if this is the case, look in the backup log for errors like:

Error returned while creating the volume shadow copy:800423f4
Reverting to non-shadow copy backup mode.

Also, the event log contains a VSS error which looks like this:

Event Type: Error
Event Source: VSS
Event Category: None
Event ID: 6004
Date:  2/05/2004
Time:  11:00:20 PM
User:  N/A
Computer: YOUR_COMPUTER
Description:
Sqllib error: Database YourSqlDatabase is not simple.

For more information, see Help and Support Center at
http://go.microsoft.com/fwlink/events.asp.
Data:
0000: 53 51 4c 53 4e 41 50 43   SQLSNAPC
0008: 33 39 37 00 00 00 00 00   397.....
0010: 53 51 4c 53 4e 41 50 43   SQLSNAPC
0018: 32 39 37 00 00 00 00 00   297.....

To make things work, you might try to enumerate the SQL and MSDE databases on your machine and attempt to change their recovery model to "simple". This should be enough to make VSS happy.

But sometimes it is not possible to change the recovery model, for example on SBS with some special SQL databases like the one used by SharePoint. In that case, you can still rely on NTBackup reverting to non-shadow mode.

2) Another potential cause for the 800423f4 error (that reverts NTBackup to non-shadow mode) is the lack of enough free space on the system drive. Please make sure that you have a couple of hundred of megabytes of free space on this drive.

One more thing about these mysterious VSS errors: we (the VSS team) realize these problems, and we are currently working to eliminate potential causes that might cause VSS failures in future Windows versions. While the VSS technology is currently the reliable way to do backups on Windows, there might be still some corner cases when VSS fails. We are working hard to either document or eliminate these corner cases in future service packs or versions of Windows.

Feel free to contact our PSS engineers (for formal support) if you experience any of these problems and want a quick resolution.