Answers to questions...
It really depends on your use case.
I'm looking to be able to pull out old dead drive, and replace it with new (almost up to date) drive and reboot.
The USB disk is a 1TB disk in a case.
Been considering dump and restoring to the usb disk periodically? or maybe something using LVM or maybe creating and breaking a RAID 1 array?
Constantly making and breaking a RAID1 with a USB drive is probably not helpful. It's just more wear and tear on the OS drive, because it has to be constantly read in order to mirror the target drive, and your USB drive is probably not what you want to boot off of if your system drive fails. You'd be much better off with a permanent RAID1, but if you can't do that then at least make a RAID1 to a USB enclosure which contains the same size and interface drive as your current system disk. This way if it fails you can swap in the backup drive and not have to boot from a USB drive.
Yeah I thought that would be the case, as I said above I'd like to be able to boot from the USB enclosure but the plan is to replace the failed internal with the disk from the USB enclosure.
Would LVM be any help or is it just a complication? I defiantly leaning to the nightly dump/restore into a similarly formatted disk.