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Im going to update my mid 2010 13" MacBook Pro to an SSD drive (Samsund 840 250 GB). I am currently running Yosemite on my old HHD drive and was simply going to make a bootable USB with Yosemite on it to install onto the new SSD drive once I got it into my computer. But then this mess with TRIM and kext came to my attention. So, how can I get Yomsemite onto my new SSD drive so that it will recognize it?

I've read a lot of threads saying to disable TRIM before upgrading to Yosemite but I didnt do that : ( Can I disable it now? Then make the bootable USB of Yosemite for my new SSD and install it with no problems? Somehow that seems too easy...

  • Do you want to boot from the SSD inside an USB enclosure? This would be a waste of money. Did you ever use Trim Enabler? I don't think so, so "disabling Trim" before upgrading is not an option. Besides, I NEVER recommend upgrading an OS as it tends to leave clutter. – Max Ried Nov 14 '14 at 9:54
  • Hi Max, I was recommended to make a bootable USB, then switch out my hard drives, and then use that USB to install Yosemite on my new SSD. Im very open to other suggestions though- whatever works! I just want to avoid getting this grey screen everyones talking about when Yosemite is running on a 3rd party SSD. – Chelsea Nov 14 '14 at 10:01
  • OK, yes, I can recommend this, too. You won't get any problems if you just install, copy, clone, rsync, whatever it onto an SSD. Just be aware that enabling Trim CAN cause such problems if done improperly. Nevertheless you should enable it. – Max Ried Nov 14 '14 at 10:29
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If you're running OS X on a HDD then I don't see how TRIM enters the equation, since TRIM is only for SSDs. If want to do a fresh install on the SSD, simply install it and proceed with your bootable USB, I do not see the problem.

What I did when installing the SSD was to put it in an enclosure, connect it via USB and use Carbon Copy Cloner to clone the OS and some other stuff to the SSD, while leaving the big data (movies, music, etc.) on the HDD (which I then put in the optical bay). If you want to have certain parts of you user data on the HDD, you can create a symlink for that.

If you do not insist on a fresh install of OS X this is the way to go in my opinion.

EDIT: A word on TRIM:

The issue with TRIM on OSX is that it is supported only for SSD installed by Apple themselves. So to enable TRIM on third-party SSDs, one needs the Trim Enabler app which does it for you. All was fine until the release of Yosemite. The Trim Enabler App uses a so-called kernel extension or kext (I think this is more or less the same thing as a driver, but I'm not sure). With Yosemite, Apple introduced kext signing which means no kernel extensions will be loaded which are not signed (which can be done only by Apple). Now the developer of Trim Enabler has no way to get his extension signed (Apple would not do that because if they did, no one had to buy SSDs from Apple to get TRIM, a policy which I find appalling).

The problem is that if you have TRIM enabled with the app and then try to boot a computer with kext signing enabled, it will refuse to boot since the TRIM extension is not signed. That makes it necessary to boot to recovery mode and disable kext signing from a command line.

So the only workaround is to disable kext signing altogether. This is a global setting, so it will apply to all possible kernel extensions which means you must be extra careful when installing drivers (which you probably never do on OS X) because it can't be verified they are trusted by Apple. But since the signing mechanism was only introduced in Yosemite, having it disabled does not put you more at risk than you were on Mavericks or before that.

The status of kext signing is saved in NVRAM, which can be reset accidentally (or manually). If reset, it will default to kext signing enabled and thus you will be unable to boot and must take the steps already outlined. Whatever you do, as long as you are able to boot into recovery mode, no permanent damage will arise from enabling TRIM. But if you are not comfortable with the thought of having to go to recovery and type terminal commands, I would just stop worrying about TRIM and not use it, at the expense of some longevity and performance of your SSD.

All information on TRIM in conjunction with Yosemite is here. I do not know how this applies to you since you're on a HDD, but assuming that you have an SSD running with TRIM enabled through Trim Enabler, you should disable it before upgrading.

I forgot to do that. What happened was that the installer ran successfully and I even got to the desktop afterwards, only after rebooting did the problem occur (grey stop sign at boot). I had no secondary computer to look up the commands listed in 2, but if you do, you can simply boot into recovery mode (hold CMD-R at boot) (either from your backup - which I hope you have - or from the Recovery HD if present, or the internet-based recovery), which will then allow you to open a terminal, run the commands, shut down and all will be working as kext signing will be disabled. You can then boot normally and choose to disable TRIM if you are uncomfortable with kext signing disabled (I see no reason to, since it did not exist before Yosemite) or just continue to use it.

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    Thanks for the detailed answer. I'm not a computer person so reading all the different things about TRIM and Kext got me really confused. I bought the SSD and an enclosure (to put my old HDD in after this is all done and use it as an external HD). So a foolproof way to do this is to simply: 1- Put the new SSD in the enclosure and use Carbon Copy Cloner to clone the OS and all my files to the SSD. 2- remove the HDD and install the SSD? Im just concerned that once Yosemite is on the SSD (with TRIM enabled) it will not work. – Chelsea Nov 14 '14 at 9:51
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    Short version: If you have never installed the Trim Enabler app, or never used it anyway, and if you do not need to have TRIM enabled, none of this applies to you and you have nothing to worry about. – oarfish Nov 14 '14 at 10:23
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See my answer here:

Since the release of 10.10.4, Apple now provides a new tool called trimforce, allowing users to activate TRIM also on unsupported disks. So now you can do:

sudo trimforce enable
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Simple way: Download Yosemite from App store (5,12 Go) Downloading you will asked where to install Yosemite. Replay (after connect) on the external SSD. It need between 20 and 40 minutes At the end the soft ask you if you want to import your data and from where You reply from your HDD The best way is now to installe SSD in your MBPro

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You are safe to upgrade you HDD to a new SSD. The problems with unsigned kexts will only affect users with third party SSDs, upgrading from a lower OSX version to Yosemite, and having enabled TRIM using Trim Enabler.

You have nothing you worry and, in case you decide to install Trim Enabler after the hardware upgrade, it will disable kext-signing checks during the install.

You have two major options for upgrading your hardware:

  1. Copy the existing partition/OS from your HDD to your SSD
  2. Fresh installing Yosemite to the SSD and then installing all your needed programs and copying your documents/data.

I would personally go for option 2 as it will help clean up the cruft that accumulated on one's computer overtime. It's also a good time to decide if it's worth reinstalling a bunch of apps that I probably only used once or twice.

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