Forum Question: Time Machine Back Up Problem/Issue – Please Help!

I have a June 2010 17″ 2.66 i7 UMBP which has a 500GB internal HD. I recently purchased an external HD of the same capacity so I could keep a full back up of all my data.
The internal HD shows capacity of 499.76GB with 160.24GB free. After connecting the external HD to the UMBP & doing a back up via Time Machine, it shows capacity of 499.76GB with 166.99GB free. This means there is a difference of 6.75GB between my internal HD & the back up one which to me seems a large amount.
I am not very technical & do not know much about Macs when things go wrong but I can only assume that Time Machine has not backed up this missing 6.75GB of data & would like to know why this is so & what I can do to correct it.
I have run Time Machine again a few times but the numbers still stay the same. Please can anyone else help me out here as at present, I just do not feel comfortable with they way things are & there is no way I can tell what data is missing on the back up HD – my luck is that it will probably be something(s) that might be important/critical in future!

Comments: One Response to “Time Machine Back Up Problem/Issue – Please Help!”

    12/18/10 @ 4:29 pm

    Bad move getting a backup drive that is the SAME size as the drive you are backing up. I always recommend getting one at least twice as big.
    You see, Time Machine doesn’t just back up one copy of each file. It is a “Time Machine” — it allows you to go back in time and look up versions of your file over time. So if you change a file one per day, and have been backing up constantly, there should be 30 versions of that file. You can go back to yesterday’s version or one from 15 days ago. If you create a file and never update it, then it will only have one copy of that file. The same copy for today, yesterday, 15 days ago, and so on. That file never changed, so Time Machine didn’t need to store another copy.
    So if you have 340GB of data, you will probably fill up that 500GB drive fast and Time Machine will start sacrificing old copies of files. Instead of 30 days worth of backups, you may only have 15 days. As an example.
    As for the difference, there are some files that Time Machine knows it is useless to back up — browser caches, I believe. The trash. Things like that. 6.75 is less than 2 percent of your data, not a big difference.

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