disc utility won't unmount hd

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Level 1

8 points

disc utility won't unmount hd

I am trying to erase my old iMac HD using a Command R restart before reinstalling the Sierra operating system. I have reached Disc Utility. In the sidebar it shows: Internal, "WDC WD1001FALS-40U9B0 Media", indented below is "Macintosh HD". Below this is Disk Images, "Apple disc image Media", indented below is "OS Base System".

I selected the top line "WDC WD1001FALS-40U9B0 Media". Typed "Macintosh HD", left Format, Mac OS Extended (Journaled), and Scheme, "Guid" and security as is then pressed Erase.

Error window shows "erase process has failed. Click Done to continue.

Details:- couldn't unmount disc. Operation failed.

I have tried First aid and this passed fine.

Any ideas to fix this?

iMac, macOS Sierra (10.12.5)

Posted on Jun 26, 2017 3:06 AM

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Answer 1

Top-ranking reply

SeaPapp

Level 7

26,486 points

Jun 26, 2017 4:50 AM in response to warrenw99

You cannot unmount the disk the computer is booted from. Recovery is on a partition on the WD disk. Select the first indented volume, usually named Macintosh HD, and erase that. If your computer shipped from the factory with install discs you need to boot from those and not recovery. What to do before you sell or give away your Mac - Apple Support

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Answer 2

warrenw99 Author

Level 1

8 points

Jun 26, 2017 6:27 AM in response to SeaPapp

Hi Mike,

your solution worked first time. Originally I was following the Apple support article What to do before you sell or give away your Mac and didn't understand step 2 (screenshot below) the selection of the target drive to erase. The confusion came from step 3, " If you don't see an erase button". On my computer the erase button was visible and selectable. This was not as clear a description as yours. I was able to erase and go back to reinstall in Disk Utility and complete a new install of Sierra. Many thanks, Warren.