Restoring and Attaching is a one way street
Many people often ask how they can us a database in SQL
2000 that they have attached or restored to SQL2005.
Unfortunately this is a one way street, once upgraded the only way of using a
database in a previous version is to copy the schema and data manually. You
can't restore or attach a database even if the compatibility has been kept at
the previous version.
Paul Randal has sort of blogged about this, also highlighting the fact that
you can't even restore
a database to a version with a lower build, so that includes hotfixes and
service packs
Graham has also raised the question on his blog http://sqlblogcasts.com/blogs/grahamc/archive/2007/05/21/sql-server-2005-database-back-to-sql-server-2000.aspx
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