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not sure which hard drive to format when trying to do complete format and reinstall

I want to wipe my hard drive and do a complete reinstall of mac os.


After I hold command+R after restart, and then go to disk utility, I see the following hard drive information as shown in the images below.


I'm unable to proceed because I'm not sure which one I shall select to format...


I don't know why there are two Macintosh_HD, both with zero space available. And the Disk Image macOS Base System is 2GB in size...


Please kindly advise what I shall do next in terms of :

  1. which hard drive, or image, to select to erase
  2. which file system I shall choose for formatting (APFS or Mac OS Extended [Journaled]) ?


My iMac info is:

  • iMac, renina 4k, 21.5-inch, late 2015
  • macOS Catalina ver 10.15
  • 1.02 TB Fusion Drive, named Macintosh_HD, as shown in "system information", with about 300GB available.


Thank you.


The first Internal Drive:


The second Internal Drive:


The Disk Image:



iMac Line (2012 and Later)

Posted on Oct 25, 2019 10:27 PM

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Question marked as Top-ranking reply

Posted on Oct 26, 2019 1:51 PM

Boot into Internet Recovery Mode using Command + Option + R. Then launch Disk Utility and click on "View" and select "Show all devices". Select the physical drive to erase as GUID partition and APFS (the top option). Once erased, quit Disk Utility and "Reinstall macOS". This will allow you to perform a clean install of macOS 10.15 Catalina.

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Question marked as Top-ranking reply

Oct 26, 2019 1:51 PM in response to oatcracker

Boot into Internet Recovery Mode using Command + Option + R. Then launch Disk Utility and click on "View" and select "Show all devices". Select the physical drive to erase as GUID partition and APFS (the top option). Once erased, quit Disk Utility and "Reinstall macOS". This will allow you to perform a clean install of macOS 10.15 Catalina.

not sure which hard drive to format when trying to do complete format and reinstall

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