[GUIDE] How to fix "Unable to unmount volume for repair" (Noob and grandma friendly)

[Nonta72]

 

Recently, I have been having weird issues installing Sierra to my newly created partition. It always ended up with a NASTY error message saying that it could not unmount the volume for repair (what repair, anyway?). At the first, I thought the volume was dirty, so I booted to my working El Capitan again and had formatted the partition again. It still failed. Then, I thought it was the fact that I was using the installer in El Capitan to install it to the new partition. So, I created an installation USB. The problem surprisingly appeared!

 

The problem:

 

The problem I believe was due to the createinstallmedia tool. If the USB is in fact created with that tool, the installer divides the installation process into two. First, it copies the installation files to the HDD partition (that part creates problem), and then it reboots from that partition to install the files and eventually create the Recovery partition. The problem is that it for some reasons tries to unmount the partition it has booted from, which is obviously not possible; hence, it fails. The root problem probably has much to do with the partitions structure. In my case, I used MiniTools Partition Manager to migrate my Windows installation from my HDD to the new SSD drive. It could have been due to that, although MSR partition is also known to cause problems in some situations. The partition structure was as follow:

• ESP = EFI Partition
• MSR = Microsoft Reserved (culprit, too?)
• Win10 = Windows drive
• Sierra = New partition created for Sierra
• Macintosh HD = El Capitan drive (already installed and working)
• RecoveryHD = El Capitan recovery drive.

 

The solution:

 

Use the "Restore BaseSystem.dmg" method to create the installation USB.

Follow these steps:

1. I assume that you have downloaded the installation from Mac App Store. If so, then right click on the file, click "Show Contents Packages". Click on "SharedSupport". You should see InstallESD.dmg. Double click on that file to mount it.
2. Open the OS X Install ESD partition. Make sure that hidden files visible (there are many ways, use Google). Right click on BaseSystem.dmg and click open to mount it.
3. Open Disk Utility. Plug in your USB drive. Click "Partition" and create too partitions. One with 200MB and the other one with whatever size you want. The 200MB partition format should be MS-DOS (FAT) and give it CLOVER as name. The second one's format should be OS X Extended (journaled) and give it USB as name. And click "Apply" to format the USB. Of course, everything on that drive will be destroyed. It's needless to say that you should backup its contents first.
4. While still inside Disk Utility app, click the second partition (it's should be named USB). On the status bar, click on "Edit" and choose "Restore". You should see "Restore from: OS X Install ESD" click on "OS X Install ESD" and choose "OS X Base System" from the dropdown menu. Now click "Restore". You might need your password to complete this task. If so, type it in and click ok then wait.
5. Upon finition, smartly unmount OS X Base System disk under "Disk Images" in Disk Utility.
6. Open the USB drive (the name "USB" will become "OS X Base System", that's why you need to smartly unmount the other one using Disk Utility to avoid confusion.)
7. Inside the USB drive, go to "System" then "Installation" remove the file named "Packages". That's actually an allias (or a shortcut if you want).
8. Go to OS X Install ESD drive. Copy "Packages" folder from there and paste it in your USB/System/Installation.
9. The same way, copy the other files from OS X Install ESD drive to the root of your USB (the root, not inside a folder on the USB drive). You need to typically copy the following files: AppleDiagnostics.chunklist, AppleDiagnostics.dmg, BaseSystem.chunklist, BaseSystem.dmg
10. Install Clover to the first partition of your USB drive named CLOVER (unless named otherwise).
11. Reboot from the USB drive and everything should go smooth and without this error.

This should also work for Chameleon bootloader. You might of course not need to create two partitions in this case.

 

You should probably not directly upgrade from El Capitan to avoid this problem. If it happens while you were trying to do an upgrade, you might need to do these steps inside a virtual machine or a working hackintosh.

[gabrielsousa]

thanks a lot . great help