Hard link

A hard link is a second name for the very same file on the same disk. Both names show the same contents, which are stored only once; change it through one name and the other sees the change. Deleting one name leaves the file reachable through the other.

You rarely make hard links yourself, but some tools — especially developer tools — use them to avoid storing identical files twice.

When Storage Studio moves a folder between your Mac and your external drive, hard links inside what you move stay hard links. That matters for space: a copy that turned each name into a separate full file would take more room on the drive than the folder took on your Mac. A drive formatted as exFAT can't store hard links.

See also: What a move keeps · Sparse file