Only connect
NOT MANY RECENT Macs can accept internal storage upgrades: it’s officially only possible with the Mac Pro, though feasible with an iMac given enough time and skill to remove and replace the display for access. Some older models have standard 2.5–inch SATA hard disks, which can be replaced — ideally with an SSD (referred to in macOS as “Flash Storage”) for a significant performance boost as well as higher capacity.
Otherwise, external drives are the answer. As we keep reminding you, you should already be using one for Time Machine backups; you could keep more files in a separate APFS volume on the same drive, but then you’d want somewhere to back those files up to… So really, it makes sense to add a second external drive. After your first full backup, Time Machine won’t need to write a huge amount, so a hard disk is fast enough; 5GB is currently a sweet spot for value, at about $100, and ideal to back up a Mac with up to 2TB capacity plus another 2TB drive. For MacBooks, connecting two or more drives to a small USB hub makes it simpler to leave them behind and plug them back in as necessary. You’ll certainly need the hub if your model has only one or two USB–C ports.
PASS THE PORT
Your external storage drive can be a hard disk too, if speed isn’t critical. If it is, for tasks like video editing, a USB 3 SSD capable of maxing out the 10Gbps USB 3.2 Gen2 interface, such as Samsung’s T7, costs $189.99 for 1TB, while slower Gen1 drives are a little cheaper and still about triple hard disk speed. USB–C/Thunderbolt 3 ports on recent Macs support Gen2, while older Macs with only USB–A ports are Gen1 (5Gbps) and won’t benefit from faster drives. Although Thunderbolt 3 theoretically supports the even higher speeds of NVMe SSDs like those inside Macs, almost no external drives exist yet to take advantage.
For older MacBook Pro and Air models with USB–A, a more makeshift but potentially convenient option is a very small USB stick, such as SanDisk’s Ultra Fit. At $18.49 for 128GB, and compact enough to leave plugged in when carrying your MacBook, it’s a simple storage expansion, but potentially slower than a hard disk and more liable to failure than an SSD.