Duplicate files · Duplicates basics
Should I remove duplicates or move files to an external drive to free space?
Short answer: Often both. Removing duplicates frees space wasted on identical copies; moving folders to an external drive frees space used by files you want but don't need daily. Clear duplicates first, then move what's big and rarely used. The window shows the exact saving first.
Often both, because they solve different problems. Removing duplicates frees space you were wasting on identical copies — nothing you need goes away. Moving folders to an external drive frees space taken by files you do want but don't need on your Mac every day.
How much duplicates free depends entirely on your files; the Duplicates window shows the exact figure for your Mac before you remove anything. A good order is to clear duplicates first (so you don't move copies you'd later delete), then move what's left that's big and rarely used. See moving folders and freeing space.