Why Apple partitions the SSD drive this way, and which partition should I removed? 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.



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?
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.