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Helpful answers
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Jul 23, 2015 4:48 PM in response to -=TheVerge=-by Kappy,What version of OS X was originally installed on your model? If it was not Lion or later, then you must first boot from the original DVD that came with the computer in order to re-partition and reformat the drive. Install Snow Leopard and update it to 10.6.8 so you can then download the appropriate upgrade for Mountain Lion or Yosemite.
If your computer came with Lion or later pre-installed, then here is what you need to try:
Install OS X Using Internet Recovery
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
Boot to the Internet Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND-OPTION- R keys until a globe appears on the screen. Wait patiently - 15-20 minutes - until the Recovery main menu appears.
Partition and Format the hard drive:
- Select Disk Utility from the main menu and click on the Continue button.
- After DU loads select your newly installed hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
- Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed. Quit DU and return to the main menu.
Reinstall OS X: Select Reinstall OS X and click on the Install button. Be sure to select the correct drive to use if you have more than one.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.
This should restore the version of OS X originally pre-installed on the computer.
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Jul 23, 2015 4:58 PM in response to Kappyby -=TheVerge=-,IIt's a Mac mini there is no did drive. But what is the difference between booting off a dad and booting off the internet?
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Jul 23, 2015 5:08 PM in response to -=TheVerge=-by Drew Reece,Are you 100% sure you are booting from internet recovery? If you are booting from the recovery partition the disk cannot unmount since it is in use by the running OS - it fits your symptoms.
Did you see the globe as it started recovery? If internet recovery can't work or fails it may fall back to a normal recovery mode.
If it continues to fail try making a bootable USB install disk - that will allow you to reformat & install whatever compatible OS X you like to the internal disk..
Either of these help with that process…
Create a bootable installer for OS X Mavericks or Yosemite - Apple Support
Here are Apple's instructions for erase and reinstall, you don't need to erase the entire HD, just the OS partition…
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Jul 23, 2015 5:17 PM in response to -=TheVerge=-by Kappy,I guess you didn't bother reading the post if all you managed to catch was "did" or "dad" when the actual acronym was "DVD." Am I supposed to be aware that you have or don't have an optical drive connected to your computer. How would it being a Mini make any difference?
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Jul 23, 2015 6:25 PM in response to Drew Reeceby -=TheVerge=-,It's definitely over the Internet, it takes 10 minutes to load. I replaced it with a new ssd meant for another computer, I guess maybe that drive is bad. I tried diskutil and even fdisk from terminal, both say the drive is busy.
FRustrating, didn't know if maybe there is another hidden option to unmount.
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Jul 23, 2015 6:28 PM in response to Kappyby -=TheVerge=-,Thanks for pointing out the ios9 beta2 keyboard autocorrect not sure why you bothered with 2 responses when this topic is definitely above your skill level.
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Jul 23, 2015 6:46 PM in response to -=TheVerge=-by Drew Reece,Just to be clear we are on the same page are you doing this in Terminal in recovery mode…
diskutil list
shows the connected disks, find 'Macintosh HD' (or the name of the internal disk) & then work out the 'device' for that disk.
diskutil unmountdisk disk5
should unmount disk 5 & all associated volumes.
I'd try a USB installer if that fails the drive is not working correctly. Try it in an external case to see if it is broken.
Grab an image if you want to show us what happens in Terminal just to be sure you are interpreting it correctly (a photo is fine).