I have a problem with internal hard drive in my IMac, it doesn't work, but I have an external hard drive (HDD), and I want to run MacOS on it.

I successfully installed MacOS and upgraded it to Sierra. The problem is that I can't update to High Sierra, or Mojave, because the hard drive doesn't have "firmware partition".

enter image description here

How can I create "firmware partition" on a simple external HDD?

I spend all day searching for info online, but found nothing helpful in my case, I would be very grateful for the help! Thank you in advance!

Here is the diskutil list log:

enter image description here

  • 1
    I assume this is the EFI partition which is used to when the firmware is upgraded. I know that High Sierra and Mojave need a firmware upgrade which occurs during the installation. It would be helpful, if you could post the output from the Terminal application command diskutil list to your question. May 9, 2019 at 17:58
  • @DavidAnderson Thank you for the response, I would be very grateful for the help on how can I create this EFI partition. I think I tried something related to this abbreviation, but would be grateful for your help, maybe this time it will work. High Sierra / Mojave installation apps won’t go further after the step when I should select the volume for installation (this is where they reject all my drives and partitions).
    – Rumata
    May 9, 2019 at 18:03
  • 1
    I can not help you without some information about how your drive is currently configured. This is why I need to see the output from diskutil list. Your question simply does not contain enough information for anyone to post an answer. May 9, 2019 at 18:07
  • @DavidAnderson Done, added the log to the description.
    – Rumata
    May 9, 2019 at 18:33
  • @DavidAnderson Also added screenshot of the error message from the installer app.
    – Rumata
    May 9, 2019 at 18:40

2 Answers 2


You must have an EFI partition (aka firmware partition) on an internal drive for the installer to accept that. You have not provided the model of your iMac, so that might be an issue. Basically, if you have one of the older iMacs, you could replace the hard drive with an SSD. 256 GB SSD are pretty cheap, have a little room for stuff and are nice and fast.

  • Unfortunately my internal drive is broken, so it’s not accessible for the installer. I’m there any solution?
    – Rumata
    May 13, 2019 at 7:59
  • I found a no-name 256GB SSD for $37 on newegg. Booting from an external hard drive is sloooow, besides, new macos versions require a firmware partition on an internal drive. So, either you get yourself an internal drive, ideally an SSD because they are really fast or you cannot install High Sierra. May 16, 2019 at 6:57
  • @user2531336 The OP has an iMac, and replacing the internal drive is "not without difficulty".
    – benwiggy
    May 17, 2019 at 8:32

There have been reports of problems when the internal drive is missing or broken, that High Sierra (10.13.4 and above) and Mojave won't install onto external drives, because it wants to update the EFI firmware partition on the internal drive.

Third-party internal replacements, which are 'blank', are also affected.

Solutions include:

  • installing MacOS onto your external using another Mac that has a working internal drive.
  • Replacing the internal drive.

There may be other ways of creating a Mojave installer that doesn't check for the internal EFI.

  • 1
    The problem with installing macOS using another Mac is that this will not update the firmware on the broken Mac. May 13, 2019 at 8:46

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