Does Time Machine back up every file every time?

Time Machine backups are unfussy: unlike many proprietary
backup software packages, you can browse a Time Machine backup
either through its interface or via the drive in the Finder.
However, it appears as if every backup is a complete backup.

This concerned Macworld reader Peter, who just purchased a new
iMac and a 4TB external drive to use with Time Machine. “This
is going to fill up my external drive fairly quickly,” he
worries.

Time Machine just looks like it’s making a full backup every
time it creates one, but it’s really using a clever technique
to only copy changed files at each hourly interval.

While Time Machine’s method has been the same since its
introduction, it’s opaque to most users, because it’s meant to
work without requiring any maintenance or fussing. (In fact,
that’s a flaw, too, because there are no tools to fix
corruption or control any of the settings, like frequency and
purging of older files.)

Time Machine only copies a file when it changes, but it creates
snapshots for each backup that use what are known as hard
links
for every file on your backed-up drive. A hard link
looks and acts like a separate copy of a file when you examine
it in the Finder or via the Terminal. So if you have a file
called House Survey 541B.docx in every snapshot in
Time Machine, you’d be forgiven for thinking that it was
duplicated all those times and Time Machine had made a few
dozen copies of House Survey 541B.docx.

However, the hard link means that the file exists uniquely just
once on a drive. Every instance of the file you see is
really a link to that one unique version. Each of
those links can be removed without deleting the original until
there is just a single link left. When that’s removed, the
associated file is deleted too.

The advantage of this hard-link system is both user-friendly
navigation in the Finder, and also a straightforward method of
restoring a snapshot without having to perform additional date
and time comparisons or other operations. It also means you can
delete snapshots without having to worry about files being
deleted that are required for other backups.

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