I can't remove a partition on my new iMac

I just had a similar problem, and referenced this support article to Apple Care.

(Fortunately I did get the Protection Plan, because I ordered my late-2012 iMac back in Nov 2012, and the standard 1 year warranty would have run out. But then I wouldn't have let the problem slide for so long if not for the extended warranty period. This partition problem didn't have an impact for me, until maybe I hit the 2TB mark...

But I digress.

So after trying do an erase from the Recovery HD partition (holding Cmd-R on boot), and also Internet Recovery (holding Cmd-Opt-R on boot), but running into the same problems as the OP (partition delete buttons greyed out, erasing the drive not doing anything useful), and being left with the EXACT same partition sizes, I got myself an Apple Care support ticket.

While setting up a time for them to walk me through the process when I'm actually home, I also tried reinstalling boot camp with boot camp assistant and removing it with that app, and reinstalling OS X, etc, in different combinations and order, to no avail.

They did reference this Apple Support article, specifically the "Additional Information - Troubleshooting" section, but I don't see what step 6 shows, which I believe is the same problem for you all here: http://support.apple.com/kb/ht5446

Their soltion solved the problem, which is in my understanding to do a hardware level delete. Even they suggest that I stay on the phone with them while I do this, so please don't go on doing the same if you're not confident, and if you don't also have the same model (late-2012 iMac, max CPU/GPU, 3TB fusion drive). Pls get a Senior Advisor, refer to this method, and ask them to walk you through the process. Here it is:

1. Go to OS X Internet Recovery by holding Cmd-Opt-R during boot up.
2. After the Earth stops spinning and the app finishes loading, the 4 options will pop up (Time Machine, Reinstall OS X, Disk Utility, Help... I think). Ignore it for now, or close it if you can't proceed to the next step.
3. Go to Terminal from the top menu bar
4. Type in "diskutil CoreStorage list", and you might need to drag the Terminal window edges a bit to see the whole table that pops up.
5. You should see a table listing your drive volumes under a title "Logical Volume Group <serial number>". It should be above a sort of double underline made from "=" signs. Copy the <serial number>, make sure it's the Logical Volume GROUP's.
6. Type in "disk CoreStorage delete <serial number>" (without the "<" & ">" signs), and it will start to self erase itself. Should take a minute only.
7. Go to the Internet Recovery version of Disk Utility, letting it load up and detect your *new* disks. An option box should pop up listing your fragmented volumes, and ask if you would like to Ignore or Fix them. The top drives on the left side panel will be red. Click Fix and wait for the magic. Check afterwards that the drives are listed like as new. Close Disk Utility.
8. Go to the option to Reinstall OS X, and let it download the files required. It should be OS X Mountain Lion if you bought your late-2012 iMac before Mavericks came out. Let it run the download & install for a while (approx 20-25 minutes each if connection's good), then update to Mavericks if you want, and start reinstalling any apps you would like. (Good opportunity to keep things to a minimum and do some spring cleaning while you're at it.)

So there is it. Hope it helps. It certainly was frustrating for a confused me =)

Pls feel free to point out mistakes if you spot them, so that this article may hopefully help others in a similar situation. Still don't know what caused it other than by removing Boot Camp...

Best,

Joe

Now I'm having the grayed out options problem with a MBP with 768ssd. I created a second 30 GB partition for the Yosemite beta and installed the beta within the partition named Yosemite. I haven't had time to mess with Yosemite beta; so I wanted to remove the partition. Now the disk utility page that used to show something like "768 ssd" followed by "Macintosh HD" shows "Yosemite" followed by "Macintosh HD" followed by another "Yosemite" and the + and - are grayed out so I can't delete the 30 Gb partition. Would Somenothing's 8-step checklist be the way to get rid of the small Yosemite partition. Then how can I rename the top Yosemite entry (above macintosh HD)? I was able to rename the Yosemite partition in finder, but the top entry is still Yosemite and everything is gray.

Ideas?

Thanks,

Richard

To my knowledge you can't remove top partition with Mac OS X Disk Utility. You have to start at the bottom removing that last partition first then work your way up. There are some 3rd party utilities that say they can do it but not with Disk Utility.

To renamed a partition in DU you have to use the Erase tab and give it a different name.

Is this on a Mac that has a Fusion drive setup? That is a combo of a rotating HDD and a SSD making both independent drives look like one drive.

Fusion drives are a whole different ball game.

It turned out simpler than I thought. I went to the local Apple Store with this support community page and disk utility open and showed the genius the problem. He tried a few things, then said if I had a backup (yes) he would wipe the drive and I could go home and restore. Then he looked again at the Yosemite 30GB partition and asked if I'd tried to erase it. When he clicked "erase" on the 30Gb partition, the top entry went from Yosemite to 751.28 GB APPLE SSD etc. Then we resized the Macintosh HD partition to maximum and were done. I just have a SSD not a fusion drive. He agreed that the fusion drives are a different beast but something similar may have caused the top line to pick up the small partition's name and cause the gray out problem.