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Disk Utility Errors and Questions for macOS 10.11 and higher

This article covers  many of the common error messages and questions you might encounter while using Disk Utility in macOS 10.11 and higher. This is not an exhaustive list of error message so you may not find your specific error here.

Note: Reformatting the drive will erase all data on the drive, so you should copy any data you want off the drive prior to formatting. 

If you are running macOS 10.13 (High Sierra) it is important to change the view in Disk Utility. Change the view to Show All Devices instead of the default which is Show Only Volumes. This only applies to macOS 10.13, so if you are running 10.12 or 10.11 then this does not apply.

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You should see at least two, sometimes three, entries for your drive depending on how it is formatted. The non-indented entry is the physical disk which typically has Media in the name. Selecting this entry should resolve most formatting issues.  The indented entries are the volumes. 

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This is important because when erasing the drive you will get different options depending on whether you have the volume or disk selected. Which is why selecting the non-indented entry is important when formatting the drive. 

What is that 134.2 MB volume on the drive?

This volume exists because the drive is setup for Windows using GUID/ NTFS.  It  is shown as Microsoft Reserved in Disk Utility. This is typical and not an issue when using the drive as NTFS. For more technical information see Microsoft documentation - here. If you plan on using the drive for Mac only then you will want to erase the disk. If you will be going between Mac and Windows then there should be no issues.

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Error Message - MediaKit reports not enough space​

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This error can occur when either erasing or partitioning the disk. It’s caused by Apple trying to use the 134.2 MB Microsoft Reserved volume for their EFI volume. Apple requires at least 200 MB for this volume which is why this error occurs. For more technical information see Apple documentation - here.

There maybe other ways to resolve this issue but the easiest way is to erase the disk and set it up with a GUID scheme. This will delete all existing volumes which will allow Apple to create their volumes with the correct amount of space. Remember if GUID is not an available option then Show All Devices in the upper left is not selected as shown in the beginning of the article.

Why is the partition button grayed out?

This is because the drive is MBR which is typically used for disks less than 3TB. This was first seen in macOS 10.11 (El Capitan) and has continued since. If you hover over the partition button you can see the tooltip provided by Apple.

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To resolve this, you must first erase the disk and set the Scheme to GUID. If you are using it for Mac only then select a format specifically for macOS. For example  Mac OS Extended (Journaled) or APFS. If you need to go between Mac and Windows then you would want to select ExFAT.

Error Message - Couldn’t unmount disk

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This error can be caused for a few reasons. If you have something open on the drive like a Disk Image or if something else is keeping the drive busy, you may see this error. Eject the Disk Image and try formatting again.

If you believe there is nothing keeping the drive busy then simply try to format the drive again. If that does not work then highlight the volume and click unmount. With the volume unmounted it should allow the process to complete.

If that doesn’t resolve the issue then try shutting your computer down, then unplug the disk and reboot with the drive disconnected. Once rebooted, reconnect the drive and try to format the drive again.

What to do if nothing else is working?

If you have access to another Mac then try the Erase process there.

If you have access to a Windows machine then you can try to diskpart the drive there. Instructions for diskpart can be found - here. Then bring it back to your Mac and try to erase the drive again.