During ArchiveOne installation you can select to install a managed version of SQL Server 2008 R2 Express. When selected, the ArchiveOne service account, by default named ArchiveOneAdmin, is automatically given the sysadmin role in the installed SQL instance.
If you want to use an existing SQL Server instance, Barracuda recommends assigning the ArchiveOne Service account either the sysadmin role, or that you pre-create the database, and assign the database dbcreator role to the ArchiveOneAdmin account. When the ArchiveOne Services starts, the service checks if the required databases exist by attempting to create them. If database creation fails because the named database already exists, the ArchiveOne proceeds and automatically creates the necessary tables, views, sprocs, etc. if they do not exist. In some instances, after an upgrade some changes may be necessary to the database schema, and ArchiveOne needs sufficient privileges in order to make such changes.