I admit to having not read all 99 previous pages of this thread.

Instead of using tar or rsync for a full drive backup, I would use an imaging solution like Clonezilla. With this, you select either the whole drive or one or more partitions on the drive. No problem with excluding external drives. And if you need to restore, you can restore to bare metal - you don't need a working installation, just the Clonezilla disc.

There are a couple of gotchas: for example you must not mount the external drive first - Clonezilla will make you select from unmounted drives to save the image on.

You can download the Clonezilla Live CD, or certain other CDS that include it like PartedMagic.