Don’t leave your USB stick/HD (with data on it) permanently connected to the computer!
Why? Because you can get an encryption virus that encrypts all your personal files, and any files on disks readable and connected to the computer (USB drives and network mapped devices included). You won’t be able to unencrypt them unless you pay the ransom which is often £500-£1500, complex to do and not guaranteed to work (you’re dealing with nasty people).
The best practise is to have two USB hard drives, costing about £45 each, and one connected permanently backup up continuously, and one that backs up once a week, or month, and stays removed to avoid getting infected with an encryption virus – only connected when doing a backup. So worst case scenario, you have only lost data from your last back to the drive you keep disconnected.
A good cloud backup solution is also a good idea. So will keep ‘previous versions’ of your files, but some just mirror what is on the computer…and could overwrite your good files if they were to get encrypted.
If you want some advice, just ask.