Still waiting for root device on an iMac

I've had fairly good luck cloning Macs using Disk Utility and a Thunderbolt cable. Recently I ran into one stubborn one that wouldn't boot up properly after the re-image.

It went straight to a flashing Do Not Enter symbol. That was a bit confusing. Was there a physical connection problem with the drive? I could boot into recovery mode (Cmd-R), and that was fine—and in recovery mode I could see the newly-imaged partition. Rebooted again. Same deal with the Do Not Enter symbol. Maybe it was just a bad image.

Imaged from another machine. Same deal. Let me boot into verbose mode (Cmd-V) to see what's going on.
I got Still waiting for root device repeated. Same deal if I tried to boot into single-user mode (Cmd-S).

When I Googled "still waiting for root device," all the top results had to do with Hackintoshes and not native iMacs. even if I Googled "still waiting for root device imac," all the top results have to do with Hackintoshes. Not very helpful, since this is a genuine Apple computer I'm working with.

Just on a lark. I held down the Option key at boot-up to see what drives were available. This actually got me somewhere, because I saw two options—Macintosh HD and the name of the partition I had just imaged. Now usually (and this is how it appears in Disk Utility in recovery mode) there is a larger disk called Macintosh HD with a partition underneath it. When that's the case, holding down the Option key at boot-up will show you only the bootable partition underneath.

When I tried to boot to Macintosh HD, I got the Do Not Enter sign again.

When I booted to the newly created partition, it booted just fine, though.

I had to explicitly select it at boot, though, every time. So I knew something was funky with the filesystem.

So I thought maybe I could just change the Startup Disk to use the working partition. When I tried to do that I got an error message that said you can't change the startup disk to the selected disk building boot caches on boot helper partition failed. None of the Google searching on that phrase helped me fix the issue.

I tried a fresh reinstall of OS X through recovery mode, but that failed:

Then I tried to see if I could verify and repair the disk using Disk Utility: mp_boot_os_x_could_not_be_repaired
It said, after I tried to repair it that I had to repair it. Thanks.

So I tried repairing it using recovery mode. mp_disk_needs_repair
Same deal. And same deal using the iMac in target disk mode (T) and then running Disk Utility repair on it from another Mac.

No matter what I did—trying to repair the drive, or to delete and re-create partitions; the partition table was super messed up, for whatever reason.

Now, I don't know if this is the solution, but it ended up being a working solution, even though it took a really long time! What I ended up doing was—instead of using Disk Utility—straight up copying entire partitions off another iMac to get the partition table corrected.

So I put the troublesome iMac in target disk mode. Same deal for a working iMac. And then I had a third iMac I hooked each of those into using Thunderbolt cables.

In the terminal, I used

diskutil list
to get all of the actual disks/partitions in question. There were three listed for each. I started with the top and just went down—copying disk8 to disk5 and then disk7 to disk4. Here's an example:
sudo dd if=/dev/disk7 of=/dev/disk4 bs=128m conv=noerror,sync
Be very careful using the dd command. It can be very destructive if you don't know what you're doing. if is the input filesystem and of is the output filesystem. You won't see any visual feedback on the progress, but you can press Control-T (not Cmd-T) to see it from time to time.

Overall, across a Thunderbolt connection, it took about two full days (48 hours) to complete a 1 TB drive transfer... but it worked! (Your mileage may vary.)

P.S. Once it was done, I was able to boot into single-user mode, run an fsck to clean the filesystem, and then reboot into normal mode and verify the disk from Disk Utility with no errors.

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