Copy Wizard Failes with 'The database principal owns a schema in the database, and cannot be dropped'

I need to setup a scheduled copy of a database in SQL 2005 SP3.

When I use the Copy Database wizard it fails with the followig enries in the log:

OnError,XSERVER,FG\Administrator,XSERVER_SSH_XSERVER_SSH_Transfer Objects Task,{CDnnnnnn-167C-48F8-B52C-8413B5937153},{219A71B7-E6B4-4512-A29C-24310A244A87},31/10/2007 13:23:49,31/10/2007 13:23:49,0,0x,ERROR : errorCode=-1073548784 description=Executing the query "IF  EXISTS (SELECT * FROM sys.database_principals WHERE name = N'tmt')
DROP USER [tmt]
" failed with the following error: "The database principal owns a schema in the database, and cannot be dropped.". Possible failure reasons: Problems with the query, "ResultSet" property not set correctly, parameters not set correctly, or connection not established correctly.
 helpFile= helpContext=0 idofInterfaceWithError={8BDFE893-E9D8-4D23-9739-DA807BCDC2AC}
StackTrace:    at Microsoft.SqlServer.Management.Dts.DtsTransferProvider.ExecuteTransfer()
   at Microsoft.SqlServer.Management.Smo.Transfer.TransferData()

I cannot see why a simple copy of a database would need to drop users.

Any help would be much appreciated as I'm currenlty on  site, scratching my head and swearing lots!