For a backup on a clustered deployment the derby database must be running on the administration server as the backup script used the configured JDBC connection to read from the database in order to create the backup XML file.  To ensure data integrity during the backup you should disable the admin.ear file.

For a restore on a clustered deployment the existing database is destroyed and a new one is created.  The restore script access the database directly on the filesystem and needs an exclusive lock to do the restore.  The admin.ear file should not be running during the restore or else the exclusive lock will fail.  The web tier and business tiers also read the new database on startup so they should be disabled during the restore so that they will get the new database startup.