I bought one of these - or an equivalent (plenty available)
https://www.ebay.co.uk/itm/USB-3-0-to-S ... SwXEdaV0q5" onclick="window.open(this.href);return false;
and a SATA 2.5" hard drive with enough storage space to accommodate my system and a little extra for growth. If you don't want to fork out for a new drive second hand ones can be picked up fairly cheaply off ebay.
Then I downloaded the free version of Macrium Reflect, which is a program you can use to clone your system drive. It is very simple to use, but there are plenty of youtube instructionals that walk you through the process of cloning a laptop drive with Macrium.
https://www.macrium.com/reflectfree" onclick="window.open(this.href);return false;
Basically, you just plug in your new drive using the USB adapter and then turn on your computer. The computer will recognise the new drive as a second drive (although you might have to format it if its a new drive). Then you run the Macrium program and clone your system onto the second drive. Turn off the computer when the cloning has finished, unplug the second drive and keep it safe. If things go tits up you simply remove the drive from your computer and replace it with the cloned drive. Then you plug in the drive that had problems with the USB adapter and make a new clone that can be used as a backup in the future.
It sounds complicated, but it really isn't. The macrium software is very easy to use and you don't need two drives of the same size and by the same manufacturer for it to work (as long as the drive has enough space to fit your system on it you will be ok). Using a drive from a different manufacturer can also make it easier to identify which drive is which in the macrium software.
It's a cheap solution that works well as a back up for emergencies. However, the clone is only a replication of your system when it was made. So if you want to keep a more up to date backup you need to plug in your backup every couple of weeks and create a fresh clone.