Firstly, no media lasts forever. Worse, backups that are safe often become unreadable simply because supporting software changes and or is dropped by the mainstream manufacturers. It is best to use something that is mainstream such as a SATA HDD. Even that, though, may disappear in the near future, so you always need to be aware of changing standards and to potentially move your backups to new media. A backup drive works best if it is not in your PC, so if your main PC fails it doesn't take out the backup drive at the same time. A network storage or USB device is a great idea. Ideally these should only be connected when needed, bearing in mind though that in theory they can only be disconnected for five years or so before they may lose data. In general it's a good idea to replace backup drives every five years or so anyway to avoid data loss, ageing hardware and dated firmware. So, a USB drive is a good plan. Just get a new one every five years.
A cheaper option is a USB writeable DVD. The disks last a fair time and you can buy an external writer for £20. The only minor issue with these is that sometimes you find that disks created by one machine can't be read by anything else. So you think you are making backups when in fact you aren't. If you want to try that route then really you need access to some DVD machines to check the disks you are creating a widely readable. Also if you smoke the rewriters don't last very long.
