IO is frozen for snapshot / IO is thawed

Following errors registered on a SQL Server Errorlog:

19-12-2006 23:49 :58.27 Database USEDDB:IO is thawed
19-12-2006 23:49 :52.04 Database USERDB:IO is frozen for snapshot

It seems the problem would caused by

1. EMC frame tools to create an SRDF to copy to our disaster site.
This occurs on SAN while the snapshot backups take place.
While databases are 'frozen' they are essentially offline and no users will be
able to connect.

2. NTBackup. This program has a commandline switch called /SNAP.
Whenever a ntbackup, even to backup 2 simple text files, all open files are frozen for a moment. Somehow, if your machine contains a SQL Server instance, a dump for all databases is created to a virtual device.

Also refer:
Error 800423f4 appears in the backup log file when you back up a volume by using the Volume Shadow Copy service in Windows Server 2003

The Ntbackup.exe operation may break the differential backup chain of a SQL Server database and may invalidate the differential backups when you perform a snapshot backup of the database