Why am I getting an error "The RAID set for database <xxx> is missing member number <x>. Backup or restore operation terminating abnormally"?

A. Probably you've used the EM gui to back up the database. You have probably accidentally left more than one backup device in the destination list in the backup screen.

What SQL Server will then do is stripe the backup to all devices. Therefore all backup devices need to be there for a subsequent restore.