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How does Time Machine under Mac OS X handle situations where the total amount of data that needs to be backed up is larger than the Time Machine disk itself?

(In my particular situation, I've got about 25 GB of data beyond the total size of my backup disk, and yet Time Machine continues without issue.)

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up vote 4 down vote accepted

Speaking from experience, Time Machine will resort to erasing your oldest backup(s) to make adequate space that it needs for your most recent backup. You always get a warning first when space runs out, but the next time you start TM up it will resort to the behaviour I mentioned above.

This topic is discussed in many forums, for example: Apple Discussions - Topic : time machine disk full

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