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.

enter image description here

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?

  • 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
  • 1
    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
  • 1
    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

1 Answer 1


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.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .