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User profile for user: barmarlis
barmarlis Author
User level: Level 1
5 points

IMac (late 2015) 21.5" Retina display needs OS installed on external drive. How do I do it?

I need to install OS on external drive, as my hard drive is failing according to EtreCheck. They recommended putting OS and my programs on an External Drive. Which keys do I push when restarting iMac to get to the screen to install OS? I think I can get the other programs on with Migration Assistant, unless someone has a recommendation for a better way. I am currently running Big Sur

iMac Line (2012 and Later)

Posted on Feb 19, 2021 9:28 AM

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User profile for user: r_scheid
r_scheid
User level: Level 3
881 points

Posted on Feb 19, 2021 9:35 AM

  1. Connect the external hard drive to your Mac.
  2. Launch Disk Utility (press Cmd + spacebar and start to type Disk Utility).
  3. Select the External drive in the sidebar (you have to select the drive and not the volume so be sure to change the view pulldown menu to "Show all devices"
  4. Click on Erase (you need to erase the drive so you can reformat it properly).
  5. Give your drive a name such as "External macOS Big Sur"
  6. Choose APFS as the Format.
  7. Choose GUID Partition Map as the Scheme.  
  8. Click on Erase.
  9. Then click Done.
  10. Get the macOS install files from the Apple Store (download Big Sur)
  11. Run the installer.
  12. A window should open (check the Dock if now). It will invite you to set up the installation of macOS Big Sur, click Continue.
  13. Agree to the software licence agreement.
  14. Next, click on Show All Disks.
  15. Select the external drive as a destination
  16. Click Install.
  17. To run macOS on the external drive hold down Option/Alt during startup.
  18. Pick the drive you wish to use to start up the Mac.


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User profile for user: r_scheid
r_scheid
User level: Level 3
881 points

Feb 19, 2021 9:35 AM in response to barmarlis

  1. Connect the external hard drive to your Mac.
  2. Launch Disk Utility (press Cmd + spacebar and start to type Disk Utility).
  3. Select the External drive in the sidebar (you have to select the drive and not the volume so be sure to change the view pulldown menu to "Show all devices"
  4. Click on Erase (you need to erase the drive so you can reformat it properly).
  5. Give your drive a name such as "External macOS Big Sur"
  6. Choose APFS as the Format.
  7. Choose GUID Partition Map as the Scheme.  
  8. Click on Erase.
  9. Then click Done.
  10. Get the macOS install files from the Apple Store (download Big Sur)
  11. Run the installer.
  12. A window should open (check the Dock if now). It will invite you to set up the installation of macOS Big Sur, click Continue.
  13. Agree to the software licence agreement.
  14. Next, click on Show All Disks.
  15. Select the external drive as a destination
  16. Click Install.
  17. To run macOS on the external drive hold down Option/Alt during startup.
  18. Pick the drive you wish to use to start up the Mac.


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IMac (late 2015) 21.5" Retina display needs OS installed on external drive. How do I do it?

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