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I upgraded to an SSD earlier this year, and the experience has been great!

I use Time Machine for backup on a weekly basis on to an external hard drive and have noticed that the size of the backups have grown ~5X. Earlier, with a hard drive, the size of the backup was ~10GB, and now, the backup size is ~50GB.

I suspect this has something to do with the way SSDs work, specifically something about SSDs having to rewrite huge sections of memory even if only a part of the section needs to be changed.

Has anyone had a similar experience?

Hardware: I am using Micron's Crucial BX100 SSD on an early 2011 MBP running OS X El Capitan (Version 10.11.6)

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  • What is the size of your current SSD compared to the hard drive that it replaced? Sep 13, 2016 at 22:47
  • HDD was 500GB and the SSD is 250GB. Current free space on the SSD is ~70 GB Sep 14, 2016 at 5:22

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I suspect this has something to do with the way SSDs work, specifically something about SSDs having to rewrite huge sections of memory even if only a part of the section needs to be changed.

This happens at the storage and controller level - it is completely transparent to the operating system.

You have already found the main cause, but one thing you didn't notice was that the day after you changed your drive, Time Machine did a complete backup from the beginning. TM noticed a new device ID on the drive and made a new backup. It is possible to join backups IF you do it before the first run after the drive change, but it involves some tinkering in the Terminal.

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  • Thank you Peter for clarifying the OS vs controller role..always good to know the proper detail :) Sep 28, 2016 at 14:34
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Discovered later that huge backup size was being created by a VM that I am also running on my Mac. Stopped backing up the VM directory and things are good.

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