Yosemite Disk Utility borked my homemade fusion drive

[neilw]

Well I'm in a bit of a pickle. I was wiping and re-installing my 2009 iMac to give to my in-laws, and the process of trying to erase my homemade fusion drive seems to have left the disk in a strange and unusable state. Before this, I was happily running Yosemite (and Mavericks before that) with the fusion drive, working fine. It's a 500 GB spinning drive with a 256 GB Flash drive in place of the superdrive (using drive doubler).

Initially I tried to do a "secure erase" with 3 overwrites, but that didn't seem to be making any progress, so I switched to single overwrite and that failed to finish with an error message that unfortunately I didn't write down. Here is the summary of the current state of the drive.

I'm booted off a flash stick imaged with a Yosemite installer.

Get info for the drive:

Name: fusion
Type: Logical Volume Group
Disk Status: Online
Total Capacity: 754.96 GB
Free Space: 41 KB
Physical Backing: disk0s2, disk1s2​

So far so good, I think.

First attached crappy screenshot shows the log of when I tried to "repair disk".

Second attached crappy screenshot shows what disk utility tells me when I try to partition the drive.

I can't make sense of all the conflicting messages. It knows it's a fusion drive, knows it's 750 GB, but will only let me have a 500 GB partition, and won't let me reformat. At one point in the repair disk log (not shown in screenshot) it said "can't repair volume while it's in use", but I'm booted off a different drive, so how is it in use.

Can anyone give me some pointers how to get this working again, and maybe some insight as to why it got screwed up this way in the first place?

 

[neilw]

In the meantime I am looking into this other thread and will see if any tips there help me. I will try fsck from the recovery partition and (if still necessary) from single-user mode and see what happens.

 

[neilw]

Well now I am getting the "prohibitory sign", circle + slash.

Booting in safe/verbose mode gives me a bunch of gobbledygook and then it finally says "still waiting for root device" (or something like that) when the circle comes up, and then the screen starts going a bit crazy, with black and white text from the verbose mode showing up.

Again, this machine worked perfectly until I tried to erase the disk with disk utility.

Anyone have any suggestions whatsoever?

 

[simonsi]

My guess would be Yosemite made it a Core Storage volume, I had this issue but can't place the terminal commands to fix it - there are threads on here with the commands from memory but have a search.

You will need to get a terminal window then run the commands to remove the Logical volume group, then the logical drive group, then DU can work as normal.

If you can't boot to a terminal window then you will first need to Option-boot from a TM backup drive or make a bootable USB stick on another Mac.

 

[neilw]

Things have now gotten worse: I cannot now boot from a USB drive. When I boot with the option key pressed, it pulls up two options: Fusion and Fusion (yes, the same thing twice). My connected bootable USB drives are not listed. So I'm feeling pretty hosed.

I am running out of options... I think I'm going to have no choice but to open the machine up and... I don't even know what. Remove the SSD? I don't relish doing that, and frankly don't even know if it'll solve anything.

I'll have to see if I can get the machine to start up in target disk mode, but I don't have any other Firewire-capable machines right now; would need to get the TB-Firewire adapter for my new iMac.

It's amazing for me to think that this was a perfectly working machine two days ago.

 

[simonsi]

Open her up, pull both drives and find another machine to format them in.

No idea why it won't boot from USB....

I've never been able to see the point of this style of Fusion drive, I have both SSD and HDD in mine, I manually control what is on each drive but it doesn't carry the risk of losing everything as here.

 

[neilw]

In general I've been incredibly pleased with the Fusion drive, and got one in my new iMac as well (factory installed this time). I prefer to let the system figure out which data needs to be on the SSD.

Anyway, I've pulled the SSD and reformatted, and will attempt to do the same for the HDD once I borrow a SATA dock. Will update my progress here.

 

[neilw]

This matter is resolved-ish.

I removed the SSD and HDD and reformatted them both, then reinstalled and reconfigured the fusion drive. All was well.

Then, because I'm crazy, I tried to wipe the fusion drive again. Less than an hour in, the machine kernel-panicked, and then the drive was borked again, just like before.

The good news is that I was able to go into terminal immediately, and repair and reformat the fusion drive. Lesson learned: don't use Yosemite disk utility to wipe a fusion drive. At least on this machine.

Craziness, but seems to be fixed. What a waste of a day.