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I am planning to reformat both of my drives in my 27 Inch iMac (mid 2011), and I have a question about the way the SSD is partitioned by Apple.

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Why are there are three tiers in my SSD?

I don't want to accidentally erase any recovery partition and don’t see an explanation of these in the Apple Support website.

If I am to reformat the SSD drive, should I erase at the 'Mercury Electra' level?

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  • It would help if you can add information such as model of Mac that you are using and if you have a Fusion drive.
    – Nimesh Neema
    Jul 5, 2019 at 11:26
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    It has to do with how APFS works. If you were to format a new disk as APFS, it would be automatically partitioned this way as well. Jul 5, 2019 at 11:26
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    You say you are planning to reformat the hard drive (HDD), but the 'Mercury Electra' is a solid state drive (SSD). So, which drive are you going to reformat (erase)? Jul 5, 2019 at 11:43
  • @DavidAnderson Clarified Jul 6, 2019 at 3:46

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The WDC WD100... and the Mercury Electra are devices. They represent the physical drives that are attached to your Mac.

Devices contain one or more volumes. Macintosh HD is a volume on the WDC device. There will be other volumes on that device that aren't displayed.

The Mercury device uses Apple's new APFS format. This uses a structure called a Container to 'contain' the volumes. That's why there are 3 objects.

If you just want to wipe your existing volume of data, then erase Macintosh SSD and Macintosh HD. That will maintain any other existing volumes, such as Recovery partitions.

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