How to re-combine a HD and SSD back into a 'Fusion' drive

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Level 1

9 points

How to re-combine a HD and SSD back into a 'Fusion' drive

Hi all,

Well, this process has floored me as to how much wasted time it has created and how the problem came about in the first place.

I'm wanting to do a clean install of El Capitan on an late 2014 iMac Retina with El Capitan 10.11.6 with a 1TB Fusion drive.

Booting from an external hard drive with OS X 10.11.4, Disk Utility wouldn't let me simple erase the drive. It came up with error message 'removing logical volume from logical volume group unable to delete the core storage logical volume...'

So I started investigating into that problem and found:

http://apple.stackexchange.com/questions/136590/how-can-i-delete-a-partition-cor estorage-logical-volume-from-the-terminal

Which various other forum posts referred to as the best solution to the problem. Being aware that I hate using the terminal, I plunged right in anyway. I've been reformatting disks on Macs for 20 years without the need of terminals?!!)

I got as far as removing the existing logical group with:

sudo diskutil cs deleteLVG

and now both the HD and SSD drives appear on the desktop as empty volumes (which is fine, I didn't need to backup anything).

But now, trying to use the:

sudo diskutil mergePartitions HFS+ "Apple_HFS Macintosh HD" (and different combinations of drive locations)

results only in 'The given partitions are not on the same whole disk'.

This is what I see with the 'diskutil list; echo; diskutil cs list' command in the terminal.

/dev/disk0 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *121.3 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_HFS Untitled 121.0 GB disk0s2

/dev/disk1 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *1.0 TB disk1

1: EFI EFI 209.7 MB disk1s1

2: Apple_HFS Untitled 999.3 GB disk1s2

/dev/disk3 (external, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *500.1 GB disk3

1: EFI EFI 209.7 MB disk3s1

2: Apple_HFS Mac Master Image 2016 499.2 GB disk3s2

3: Apple_Boot Recovery HD 650.0 MB disk3s3

I have no idea how to recombine the separate HD and SSD drives into a single Fusion drive, with which I can then do a clean install of El Capitan. Can someone please help!?

Also, how can quickly and safely erase a Fusion drive in the future? I need to deploy a new master image with all our software on it another 10 times, and I want to totally erase the drives for clean installs (using Migration Assistant for about 8 years moving from system to system has resulted in a lot of weird and wacky behaviour on our Macs) and it's time for a clean install for all of them.

Again, I'd greatly appreciate any help on this.

- Alex

iMac, OS X El Capitan (10.11.4), iMac (Retina 5K, 27-inch, Late 2014

Posted on Jul 21, 2016 10:48 PM

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Answer 1

Best reply

Linc Davis

Level 10

209,547 points

Aug 16, 2016 10:25 PM in response to Helix NRG

Please read this whole message before doing anything. If you don't feel confident that you can follow the instructions, get someone more experienced to help you.

This procedure applies only to an iMac or Mac mini with a factory-installed Fusion Drive.

You'll need to refer to these instructions while no web browser is running, so either print them or load them on another device before you begin.

1. All data on the internal drives will be removed, including Boot Camp data, which is not backed up by Time Machine. You need at least two complete, independent backups. One backup is not enough to be safe. There are ways to back up a computer that isn't fully functional. Ask if you need guidance.

2. You'll need to start up from a Recovery system other than the one on the internal hard drive (if it has one.) There are several ways to do that.

a. If you have a local Time Machine backup (not a network backup), or a compatible installation of OS X on an external drive, then you should be able to start from it by holding down the option key at the startup chime. Select the external drive in the row of icons that appears. By "compatible," I mean an installation that can run on the machine. A version of OS X that is older than the machine is not compatible.

b. If you used Recovery Disk Assistant (or can use it now) to prepare a Recovery system on a USB flash drive, you can start from that by holding down the C key at the chime. No icons will appear. Startup will be much slower than usual.

c. Start up in Internet Recovery mode by holding down the key combination command-option-R at the startup chime. Release the keys when you see a spinning globe. Select a language, if prompted.

Note: You need an always-on Ethernet or Wi-Fi connection to the Internet to use the network features of Recovery. It won’t work with USB or PPPoE modems, or with proxy servers, or with networks that require a certificate for authentication.

3. Disconnect all other external storage devices.

What happens next depends on what version of Recovery you're running.

☞ If the machine came with OS X 10.10 ("Yosemite"), 10.9 ("Mavericks"), or 10.8 ("Mountain Lion"), and you're in Internet Recovery, please take Step 4a and skip Step 4b.

☞ Otherwise, skip Step 4a and go to Step 4b.

If you don't know what version of OS X the machine came with, look it up on this page.

After taking whichever of those steps is applicable, you should have an empty Fusion Drive named "Macintosh HD" on which you can carry out either Step 7a or Step 7b.

4a. Repair the Fusion Drive (see under "Troubleshooting" on the linked page.) Quit Disk Utility and go to Step 7a or Step 7b. Skip Steps 4b, 5, and 6.

4b. In the OS X Utilities screen, select Get Help Online. Safari will launch. While in Recovery, you'll have no access to your bookmarks, but you won't need them. Load this web page.

5. Triple-click anywhere in the line below to select it:

N=Macintosh\ HD; for d in /dev/disk?; do o=`diskutil info $d`; for D in SSD HDD; do eval [[ ! \"\$$D\" ]] && grep -lqw "APPLE $D" <<< "$o" && eval $D=$d; done; [[ "$SSD" && "$HDD" ]] && break; done; diskutil cs create "$N" $SSD $HDD && diskutil cs createLV "$N" jhfs+ "$N" 100%

This is a very long line, and you may not see all of it on the web page. Be sure to select the whole line, not just the part you can see.

Copy the selected text to the Clipboard by pressing the key combination command-C.

Quit Safari. You'll be returned to the OS X Utilities screen.

6. From the menu bar, select

Utilities ▹ Terminal

The Terminal application will launch. Paste into the Terminal window by pressing the key combination command-V. Wait for a new line ending in a hash sign (#) to appear. If nothing happens, press the return key.

Quit Terminal to be returned to the main screen.

7a. If you have a Time Machine backup, connect the backup device and restore your data. If you backed up to a network device such as a Time Capsule, it should be connected automatically.

7b. If you don't have a Time Machine backup, or if you can't restore from the Time Machine backup that you do have, install OS X. Restore the data afterwards in Migration Assistant.

In Internet Recovery mode, you'll be installing the original version of OS X that shipped with the machine. If you've upgraded the OS in the App Store, you'll need to do that again.

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Answer 2

Helix NRG Author

Level 1

9 points

Aug 16, 2016 10:27 PM in response to Linc Davis

This solved my problem – THANK YOU!

My big question is this however - why could I only repair the Fusion drive from the Internet Recovery mode and not from the external hard drive I was running OS X 10.11.4 with Disk Utility on?

I didn't need to restore any files, luckily, only just 'fuse' the fusion drive back together again, which Disk Utility would not allow from the external drive.

How am I to completely wipe Fusion drives in the future without them splitting with a terminal command or Disk Utility saying the disk can't be erased because it's a Fusion drive?

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