I have a 500GB external dedicated to TM. Seeing some questions on the forum a few days ago, I just checked mine and found out it is almost filled up! My computer HD has about 150GB on it so where did the extra 350GB come from? I was under the impression that TM just overwrote existing data to update.

Your impression is mistaken, TM is incremental backup by design and only when when it has used nearly all the capacity of your drive does it start overwriting existing backups to make space for the newer backups. It does this by dumping the oldest backups sequentially to clear space for the newer versions.

If it overwrote existing data, how could you go back in time to restore previous versions of files? That's not how TM works.

Yes, it will eventually fill up whatever space you assign it with the incremental backups.

The more HD space, the farther "back in time" you can go to retrieve a file. Once it hits it's limit, the oldest slice goes to make way for the newest.

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all who would be called good
must suffer with him.

Everybody matters or nobody matters.

You and I have memories longer than the road that stretches out ahead.

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by believing all possible evil of evil men.

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And hope is a lousy defense.

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It WILL dump data when it needs room. Read Time Machine Help. I believe you get notification for that.

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Ed (modelamac)

I think I will just put an OUT OF ORDER
sticker on my head and call it a day.

>f it overwrote existing data, how could you go back in time to restore previous versions of files?
So I didn't explain it properly. What I meant was I thought it would add new data but not add a whole day's data if nothing had changed. Since I do back up everything important to my externals I should be OK for work data, etc.
Thanx all.

as I understand it, first time you start Time Machine, it will back up the entire (internal) drive to the external Backup disk. It's almost like a "clone" (but it is NOT a clone). All the data is there, but it is not bootable.

Next time TM does a back, it looks at the internal drive, and the data that has changed or was added it is backup up to the external drive, the data that did not change, it is not backup up again. This is why the second back takes much less time than the first one.

Now it is possible that you have a large file somewhere (database?) that has several GB, and you may make small changes to this file, but TM will back up the entire file, and thus it gets backed up again and again and it fills the drive.