Both of the SSD's are identical. . Maybe I should try Clonezilla?
A second OS is needed since an identical disk can not be mounted with the same uuid. So a simple system with the package 'qemu-utils'. This includes all you need.
For a disk to a file
qemu-img convert -p -c -O qcow2 /dev/sda /path/to/file.qcow2
-c compresses therefore qcow2 needs to be the format
for file to disk
qemu-img convert -p -O raw /path/to/file.qcow2 /dev/sda
-p fro progress, no -c, -O chooses raw or file format
Use bleachbit or a few other utilities to zero free space from within the running system before you make the image, this will allow the compression to work better - it WILL image extra data in a dirty filesystem, deleted files, etc, so zero the free space.
My cli/ncurses OS's are ~1GB, my base OS's are as tiny as 1.2GB, as big as 20ish.
An encrypted disk's image is the disk size. Without that, a typical OS will image and compress to about 50-60% of what your running OS told you. With further prep you can compress it to 30-40%. It's highly likely your OS will pack into a few Gigs, plus all your personal data, which is why you should separate that stuff...
So the spare disk could have a simple OS and a few copies of the primary OS, and still leave room for non-OS file backup since it is NOT a copy it can be mounted and run along side the primary OS. Grub can include both disk, from both disk...