There can be a number of reasons why you may be looking at extra storage with your computer or tablet.
For instance it may be your built in memory is limited with many Chromebooks having just 32 gb or you have a great but older laptop with around 40 to 80gb of memory in the form of a hard drive.
It may be you have a more modern laptop with around 250gb or more built in hard drive which is adequate for most things with the possible exception of lots of HD video but are looking at safe storage for things like photos or music download you bought.
You do have a number of options, one being using cloud storage or image sites which is fine but you need reliable internet access to 'pull down' your documents and media.
For that reason some of us look toward 'local storage' of which one of the oldest is the humble USB memory stick which gives us a small form Solid State memory in increasing capacities but when it comes to gigabyte per pound value a mechanical portable hard drive is hard to beat
Recently I bought a new one, a Toshiba Canvio Basics series one with a capacity of 500gb and is USB 3.0 compatible for extra read/write speed with newer computers for £33.99 delivered from Amazon UK.
It's pretty barebones, no software to install with buttons to press for backing up and comes pre-formatted to NFTS for Windows (although it can be re-formatted for Apple's OSX) and is literally plug and play with your computer finding and installing the driver.
You just drag and drop what you want to copy on to it, maybe creating a folder and that's it a cheap and good answer to storage stuff safe in case your computer has a malfunction.