Both fix a full disk. One costs about $90 for 1 TB at 2026 prices and adds a cable to your desk. The other is free to check and takes a minute. The right answer depends on what is filling the drive, so find that out first.

Find out what is on the disk

macOS’s storage settings will tell you that “Developer” or “System Data” is large, and stop there. A scan that knows the tools is more useful. Cache Goblin lists every cache by the tool that made it, with its size and a grade: Safe if the tool rebuilds it, Restorable if it comes back with a download, Risky if it might hold your work. The scan is free and deletes nothing.

Two outcomes are common.

The scan finds 50 GB or more. The disk is full of caches. Clean the Safe ones with one click and go through the Restorable list. You have the space back and you did not buy anything. Most development Macs a year or two old land here.

The scan finds under 20 GB. The disk is full of your files: photos, video, datasets, virtual machines, a music library. A cleaner cannot help, and an external drive is the right purchase.

What an external SSD is good for

  • Media libraries, archives, downloads you want to keep.
  • Virtual machine disks and large datasets you use occasionally.
  • Time Machine, which should not be on the internal drive anyway.
  • Project folders you are not actively building, if the drive is fast.

What it is bad for

  • Caches. Build tools want their caches on the fastest local disk. Moving DerivedData or node_modules to an external drive slows every build, and a symlinked cache breaks in ways that are hard to diagnose.
  • Anything you need when the drive is unplugged. A laptop with half its working set on a drive in a bag is a laptop that does not work on the train.

The decision

  1. Scan first. It is free and takes a minute.
  2. If caches are the problem, clean them. Consider the drive later, if at all.
  3. If your own files are the problem, buy the drive, and put the caches nowhere near it.

Download Cache Goblin free to run the scan. If it turns out you need the drive, you will at least buy the right size.