It’s never been easy to upgrade the storage in Macs, so it’s always a good idea to buy an external drive to provide extra storage for all your work files, photos, videos, and games. And, of course, you should always have an external drive available for making Time Machine backups in case anything ever goes wrong.
Modern solid-state drives (SSD) are the best option as they’re really fast, reliable, and compact. However, SSDs are still relatively expensive, with 1TB drives costing around $100/£150, whereas that same amount of money can get you a 4TB basic desktop hard drive with a USB interface (drives with high-speed Thunderbolt ports tend to be more expensive) Hard drives are slower than SSDs, but if you need lots of storage for your important files and back-ups then a good old-fashioned hard drive is still the most affordable option. (If it’s an SSD you are after, read Best external SSD for Mac.)
There are plenty of choices, too, with compact, portable hard drives designed for use with laptops, and desktop drives with very larger capacities if you really need a lot of storage. Some hard drives also provide additional features, such as built-in docks with lots of Thunderbolt and USB ports, or perhaps allowing you to open up the casing of the drive and insert a new drive for an instant upgrade.
Read more at Macworld.com