Will a Duo v1 (RAIDiator 4.1.8) backup to a 3 TB USB?

There is no support for disks larger in 2TB in capacity either internal or external.

You could connect the 3 TB USB drive to a PC, and backup over the network. The main disadvantage is that the PC would need to be turned on when the backup is scheduled (and left on until it completes).

If you are sticking with X-Raid, your current USB drive should be ok (unless you are storing multiple backups on the same drive). After expansion, the duo's volume should be slightly smaller than the external drive size.

I'm OK with one backup set on my USB drive.

But I have the backups set to "Full backup once every week". If I select "Remove the contents of the backup destination before a full backup is performed." then there will be a short period (well, with the backup speeds I see, more like a day and a half!) when I won't have any backup (before the current full backup is completed).

If there was an option to 'Replace files on destination when doing full backup", that would be better, IMO.

You could try shifting to rsync (http://www.rnasguide.com/2010/09/23/rsy ... -usb-disk/).