Why doesn't Norton 360 allow me to select an external hard drive as a backup location?

Why doesn't Norton 360 allow me to select an external hard drive as a backup location?  It only allows Norton cloud or personal cloud drives that have been disconnected.  Is Norton locking me in to Norton cloud?

[Edit: Clarified subject]

An external drive has to be connected to your computer and has to have a drive letter assigned to it for you to be able to backup to that drive. 

 

Norton Backup Settings allows me to select my external Seagate drive.
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