The most important thing to remember about hard drives is that they fail. In fact, the seven-year-old 2TB USB drive I was using to backup my desktop PC failed on Saturday. That wasn’t a problem because my PC’s hard drive was still fine and I had my 2TB backup backed up to an 8TB USB drive. (The 8TB drive also backs up the backups to two laptops.) On Monday, I replaced the dead drive with a new 4TB USB 3.0 drive, because you can never have too much backup space, and it’s really not worth buying anything smaller.
This sounds like the sort of setup that would suit you, too.
A couple of 4TB drives would currently set you back around £174, though you could save about £33 by buying 2TB drives, or about £25 by buying 3TB drives. (I’m using Seagate Expansion external hard drives to simplify the example.)
You could buy a two-bay NAS (network attached storage) device, but it would be roughly twice the price. First there’s the cost of a good, cheap NAS, such as a Synology DS218j (£165.23). Then there’s the cost of two hard drives to fill it. A couple of 4TB Seagate Ironwolfs (£190) or fast and ultra-reliable HGST Deskstars(£229), perhaps? Total cost: between £350 and £400.
NAS drives have their uses, but they are not the cheapest way to back up a laptop.
Before you buy anything, work out what you are trying to achieve, and why. In particular, distinguish between backups and archival storage.
The original point of having a backup was to get your PC up and running again if a hard drive failed. This resulted in compressed, incremental backups. It was a good solution for people who had one desktop PC that stored all their data.