I tried to restore SharePoint 2003 portal on SQL Server 2005 and it failed with following error even though I granted db owner rights to the user:
Portal creation failed System.Data.SqlClient.SqlException: The server principal "domain\sharepointadmin" is not able to access the database "ORG_SITE" under the current security context. at Microsoft.SharePoint.Portal.Data.a.c(SqlCommand A_0, Boolean A_1) at Microsoft.SharePoint.Portal.Data.a.b(SqlCommand A_0) at Microsoft.SharePoint.Portal.Topology.Database.d(a A_0, String A_1) at Microsoft.SharePoint.Portal.Topology.Database.a(String A_0, String A_1, String A_2, Boolean A_3) at Microsoft.SharePoint.Portal.Admin.c.b() at Microsoft.SharePoint.Portal.Admin.c.f()
Investigation the issue, it ended up as a SQL server's security issue.
I granted the domain\sharepointadmin user sysadmin role on the SQL Server 2005 and problem was solved! It restored portal smoothly.