ChrisL,
I've been mulling over how to best answer this for a while.
Have to say that I'm leaning heavily in same direction as N4RPS on this. If you're really not going to use the drive very often and are concerned about it powering on when not being used, then best bet would be to put the drive in an external enclosure.
Reasons why?
1. As N4RPS stated, you can turn it on whenever you want and you can share the drive between computers if you want to as an added benefit.
2. Because it sounds like you want to clone partitions vs. just backing up certain data files, if you keep a cloned copy of the Root partition in the computer, that clone will have the same UUID as the current Root partition. That will cause problems because booting will look to both of them. If you keep a clone on computer, you need to change the UUID of that partition before trying to boot into your system again. It would also be a good idea to change the "/etc/fstab" file on the clone to match the new UUID. That way you don't have to remember to do that later.
If you go with an external enclosure, go ahead and clone the whole drive over (see note below on Windows 10 first). That will (I think) copy the MBR along with the Root and Swap partitions and it should be bootable on its own when you're done.
If you want to clone to the drive with it connected internally, then only clone the Root partition. Just leave free space available for adding Swap later if that becomes necessary. Keep a live DVD/USB handy of the same version of Linux Lite that you have on that backup. You'll need it to install grub boot loader to MBR if you need to start booting from that drive. (There is a link for reinstalling grub from live DVD listed under
this tutorial.)
Formatting of partitions? I've only toyed around with cloning a couple of time, so not 100% sure. I think the cloning process copies not only the data but the filesystem type as well, so I don't think you need to worry about that.
Windows 10? If you want to keep it, then don't do a full disk clone -- clone the partitions one at a time in partitions you create on the disk after the Windows partition(s).