Error: The primary filegroup cannot be backed up as a file backup because the database is using t (4256344)

LiteSpeed fails on the combination of Simple recovery mode + Primary file group backup (sample script below)

exec master.dbo.xp_backup_database @database = NPartialTest,
@fileGroup = NPRIMARY,
@filename = NE:\Cynthia\PartialTest200803141020LiteSpeed_File.BKP,
@backupname = NPartialTest backup,
@desc = NBackup of PartialTest on 2008-03-14 10:20:56,
@with = NSKIP,
@with = NSTATS = 10

LiteSpeed Version 4.8.4.00086

Copyright (C) 2004-2006, Quest Software Inc. All Rights Reserved.
Quest Software.
Registered Name: NEMO

Msg 62309, Level 19, State 1, Line 0
SQL Server has returned a failure message to LiteSpeed which has prevented the operation from succeeding.
The following message is not a LiteSpeed message. Please refer to SQL Server books online or Microsoft technical support for a solution:

BACKUP DATABASE is terminating abnormally.
The primary filegroup cannot be backed up as a file backup because the database is using the SIMPLE recovery model. Consider taking a partial backup by specifying READ_WRITE_FILEGROUPS.

The SQL Server parameter READ_WRITE_FILEGROUPS is not a valid LiteSpeed parameter (in GUI, script or commandline). The READ_WRITE_FILEGROUPS will only work in the native SQL Server if the other file groups are Read Only. If the file groupsare not set to Read Only you get a full backup.

Utilize native backup tool READ_WRITE_FILEGROUPS for file groups in Read Only or for backing up the Primary file group.

For additional info, seehttp://msdn2.microsoft.com/en-us/library/ms186865.aspx

Also see SOL20114, SOL44313, SOL32757for detail onREAD_WRITE_FILEGROUPS