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As it turns out, there was one item from my laptop drive failure a few weeks ago that didn’t end at all like I expected.


I pretty much did the necessary things. I backed up my drives regularly. I had a full clone backup. I made an emergency Lion USB thumb drive. I had a Time Machine backup.

When I got my laptop home, i booted up from the thumb drive, restored from my Time Machine drive and was on my merry way.

After working for the day, I went back into my backup routine. Running SuperDuper at 11:30pm, I noted that the backup seemed to be taking longer than normal. But by that time it was completed and didn’t concern me anymore.

The next day, after working a bit, I initiated a Time Machine backup and went off to handle other tasks. When I got back to my computer, Time Machine was still churning. So, I did a “Get Info” to see if my suspicion was correct. Previously, on this Time Machine backup, there was 50GB free. Now, there was 200 GB free. Time Machine was overwriting existing data. I wasn’t too happy about it, but you can’t un-ring the bell.

What had happened?

Well, actually, I’m still a little confused because of what’s now on my Time Machine backup.

Let’s start with “What didn’t happen.” With the initiation of that Time Machine backup, after the replacement of my laptop drive, I expected to see a dialogue box with a warning, question, or a confirmation. I got neither. What I got was Time Machine automatically using my existing Time Machine backup as a drive to overwrite. By the time I realized what was happening, data was already overwritten. I decided to just let it go on with the overwrite. I couldn’t un-ring the bell anyway.

That overwrite is most likely sparked by the fact that the drive in my laptop is new.

With a new drive ID (UUID), even though the info on the new drive was (old) from a Time Machine restore, the drive is still a new ‘different’ entity to Time Machine. Hence, data on the new drive is considered new data.

Is there a way to have the new drive talk to the Time Machine backup to say “Hey, I’m you. Only back up the changed content?” Yes, it seems there is.

Googling brought up this very long thread of varying advice on Mac OS X Hints:

My confusion lies in trying to figure out exactly what did happen.

Did Time Machine actually see my backup files? Yes, I believe so. Here’s why…

With all the time it took. I expected that Time Machine did a complete overwrite of the existing data on the Time Machine Backup drive. There was data back as far as 12/10. I now expected that the date inside the Time Machine Backup would read only as one backup, 12/3/11, the date of the initiation from the new drive. But it doesn’t. My Time Machine backup starts at 8/14/11, and has actual files to prove it.

So, evidently, Time Machine did see the existing data, overwriting some data but not all. Why? Might it have anything to do with the 500GB drive having only 50GB free at the time the backup was initiated? Did it think it didn’t have enough space for the laptops 203GB of content? After the Time Machine backup, I now have 200 GB of free space on the Time Machine backup drive, where there once was 50GB of free space.

I’m curious, in case this happens again. So, if you have any insight, please chime in.

Am I unhappy about the partial overwrite? Not really. The only files that I’ve thrown away over the past months are apps that have been removed completely due to upgrades. And, when I needed the TIme Machine backup for restore, it was perfect.


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3 Responses to One Minor Hiccup

  1. Joseph says:

    Doesn’t Time Machine in Lion now “work” even when you aren’t connected to your backup drive? So is it possible that the files prior to 12/3 were saved by Time Machine on your internal drive, and so it recognized them as “from this drive”?

  2. Chita says:

    That could be the reason, since I did install Lion in August.

  3. Greg says:

    Curious to see what others have to say. Time Machine is sometimes too much of a black box!