After talking to myself a lot, I am giving up on fast/large drives and sticking to SD's for now.
Maybe I am an idiot but SD Card Copy doesn't work for me.
I can back up fine but only once to a fresh USB device.
Second time it doesn't work because of the UUID conflict.

SD's are slow but fast enough to run the system and easy to backup with win32diskimager.

I have never checked that UUID box…never…
Also you can only copy a 120gb drive to a 64gb card, as long as the spaced used on the 120gb drive is less than 64gb, and also when you copy a 64gb drive to a 120gb drive, you will only be using 64gb of the space so you need to extend the space using raspi-config

as for your issues, I have only ever copied back and fourth from SD and SSD drives, never done with a USB thumb drive, but it should be no different, but like I said never had UUID checked, so ignore that, as mine have worked every time with this method…
Aslo no need to format drives first as the copy function does all that…

Hi Astro,
This really bugs me... It works for you but it doesn't for me.
I only have 16gb of data on the card/drive.
SD Card Copy just doesn't work for me with the symptoms I described.
I am able to back up once to a new/fresh drive. Second time it doesn't work.
Maybe I have crooked version of SD Card Copy?

Hmmm, the best thing is to try again with fresh cards, and don’t use the UUID setting….
I have to say, I have used it loads, and even with a full working card or SSD I just plug it in and copy to it, the formatting of the back up card / drive and everything is taken care of during the process, in fact I just updated my Stellarmate set up last night, which runs from an SSD, and when I had updated I just put my back up card in the slot, which was a full working back up, and copied the drive to it…

I think your issue stems from the use of the UUID setting….
 

One thing I do have to say, is that I have never had identical card / drives for back up and main, my system runs from an M.2 SSD and I keep back ups on SD cards….or I have had SD for main and the back up on an SSD,
so there may me an issue too….

try it with different cards or drives and don’t use UUID setting….

I have recently moved to an SSD. I have moved the "Pictures" folder to a separate partition. I went looking for a backup solution and found rpi-clone. A great script which copies partitions and creates bootable images.
To get rpi-clone:
$ git clone  github.com/billw2/rpi-clone.git
$ cd rpi-clone
$ sudo cp rpi-clone rpi-clone-setup /usr/local/sbin

I make the backup to a SD card. I am only going to backup the first 2 partitions, boot and root.
I used:
sudo rpi-clone -f2 /dev/mmcblk0

The gives me a bootable copy on an SD card which I can put back on the SSD usinfg rpi-clone if I need to.

OK. It does work but only between SD inserted in the slot and a USB.
Doesn't work between two USB devices.
Whew...
Thank you for not letting me to give up!
: )

I am sure it would work fine to another SSD drive but I have a few SD cards laying around that are now redundant so this is an ideal use for them.

I really feel its making a big difference running from SSD. Far more responsive and SD card are far more likely to suffer failures than SSD drives.