What to do when a Time Machine copy to an external drive is enormously larger than expected

It’s all about the hard links.

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Time Machine uses a clever technique to make snapshots of your Mac backups. Instead of just creating copies of files that have changed since the previous backup—incremental archiving—Time Machine in macOS creates effectively a copy of each backed up volume. The clever part is that it doesn’t duplicate any file that’s remained the same between snapshots; it just adds the newly updated files.

macOS gets away with this by using “hard links,” something introduced several releases ago in which a file on a given volume can be referenced multiple times from a single central copy. It exists just once on the volume, taking up only that amount of storage. Each reference is just a few bytes. Unlike an alias in the Finder, a hard link acts to the operating system just as if it were the original file, including when it’s modified. You only wind up deleting the file if every hard link is deleted. (Essentially, deleting all but one hard link reverts it to just being a single copy of a file.)

Now all this preamble is necessary to explain a problem one Macworld reader encountered when trying to copy their Time Machine volume to a network-attached storage (NAS) system. While some NASes provide support as Time Machine destinations, this support typically involves creating a folder or volume that acts enough like an HFS+ formatted volume to handle Time Machine’s odd requirements. (Time Machine can only back files up to HFS+ formatted volumes.)

But copying from a HFS+ drive to a NAS will typically treat all hard links as real files, duplicating them. Our reader’s 340GB Time Machine volume wants a whopping 75TB on the NAS.

The better way to proceed on this front is with a NAS that supports Time Machine: make it a destination and Time Machine will begin by creating a complete copy of your currently selected volumes to that destination. Retain your historic Time Machine backups to retrieve older versions of files or even files later deleted.

This Mac 911 article is in response to a question submitted by Macworld reader Nico.

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