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I’m trying to install Mac OS X Lion after accidentally having erased my disk in order to reboot my iMac, which I did without backing it up. On a separate USB drive, I formatted and installed a Lion DMG file in order to reinstall the software back onto my computer, which currently starts up recovery mode because of my missing OS.

In order to download my OS from my USB into my hard drive, I believe I need to partition and erase my drive, however I get the error that the disk cannot be unmounted, most likely because the volume is being used? I have tried forcing the disk to unmount several times and have been stuck on this issue for days.

Please help. I just want to use my computer normally.

  • What is the model/year of your Mac? – David Anderson Mar 30 at 9:13
  • Apple iMac "Core 2 Duo" 2.0 20-Inch (Mid-2009) – Ujwal Khatiwoda Mar 30 at 15:19
  • I can somewhat understand that you have Lion stored as a DMG file, but you can not install Lion until you have Snow Leopard installed. So what is this DMG? Is this the upgrade to Lion? Or, is this a backup of some previous install of Lion? Is would seem to me that you would be better off installing Snow Leopard and then upgrading to El Capitan. I believe this does not cost anything. – David Anderson Mar 30 at 23:09
  • I installed a .dmg of Lion OS from the web. I formatted the USB and restored disk image of the .dmg on my USB using Transmac on my Windows PC. How would I carry out installing the Other OS though? If I tried to install Snow Leopard, then does that mean I wouldn’t have to verify an Apple ID like for the Lion OS? How would I install Snow Leopard, I figure I would create a bootable USB with Snow Leopard on it and then what? – Ujwal Khatiwoda Mar 30 at 23:45
  • OK, are you saying you can not reinstall Lion from the internet because you need the Apple Id used when Lion was first installed? – David Anderson Mar 30 at 23:59
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If you were to purchase Lion from Apple, then a requirement to install would be to have Snow Leopard already installed. In other words, the DMG file only offers an upgrade to Lion. You probably can not use the DMG file to create a bootable installer. You can read the requirements from this Apple website.

You would not need a Apple ID to install Snow Leopard, but the installer will still ask. You can decline to give an Apple ID if you wish. Snow Leopard was released on a DL DVD. Apple still sells Snow Leopard DVDs, but you can also download an ISO from the internet. There are plenty of websites that explain how to make a bootable USB Snow Leopard installer. I have not tried any of these methods. If you succeed, then maybe you could post a method that works.

I would recommend burning the Snow Leopard ISO file to a DL DVD.

Once you have Snow Leopard installed, you should install all the available Snow Leopard updates. This would update Snow Leopard to OS X 10.6.8. From here, you could upgrade to Lion by using your DMG file. Or, you could open Safari and search for the Apple website where you can download a free upgrade to El Capitan.

Note: You may need an Apple ID to download El Capitan. If so, then use an existing Apple ID or create a new ID. Apple does not charge for creating new Apple IDs

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Hold down Command + R as you boot your Mac. This will cause it to boot into recovery mode. See the Apple Support document, About macOS Recovery

When you're in recovery mode, a stripped version of the OS is loaded into memory, so you should be able to format your drive and perform an install.

Mac OS X Lion is ancient at this point. It would be great to know which iMac you have and why you're still running Lion. Is it a PowerPC based Mac?

Also, do you have Internet access? Most recent Mac's will download the latest OS if you go directly to reinstall macOS. Normally that's the best bet all around.

Let me also ask...because you pretty much have to be LOOKING and looking HARD for trouble to erase your disk on a Mac, how did you "accidentally erase your hard disk"?

  • Let me give you context, I got this iMac, Apple iMac "Core 2 Duo" 2.0 20-Inch (Mid-2009), from a family member. I don’t know the Apple ID which was used to install the OS onto the device, so if I were to use the reinstall OS function on recovery utilities, it would ask me for an Apple ID and would say “This item is temportarily unavailable” after I type in my personal Apple ID. I’m pretty sure it is not a PowerPC based Mac. Also, when I got it, it had the previous owner’s data and information still on it, so I wanted to do a fresh reinstall of the OS. I understand I was being ignorant. – Ujwal Khatiwoda Mar 30 at 15:18

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