OS X Yosemite and third-party SSDs: Here's what you need to know

With OS X 10.10 Yosemite, it turned out that Apple made a change to the way that the operating system worked. This caused problems for some users who had installed third-party SSDs into their Macs, particularly if that system was a boot drive. Here's what you need to know to avoid problems.
Written by Adrian Kingsley-Hughes, Contributing Writer
Image: OWC

With OS X 10.10 Yosemite, it turned out that Apple made a change to the way that the operating system worked. This caused problems for some users who had installed third-party SSDs into their Macs, particularly if that system was a boot drive.

In order to get a better understanding of the problem, along with some solutions, I turned to OWC and its founder and CEO Larry O'Connor. OWC specialize in upgrade components and accessories for Macs, and the company has its own line of flash and SSD drives specifically for the MacBook, the MacBook Pro, Mac mini, iMac, and Mac Pro.

Q: What is TRIM and why is it important?

TRIM is effectively an interface for the OS to help manage available space on an SSD. Rewriting NAND that had prior been used to store data that has since been deleted is different that it is on a hard drive. For drives designed with a dependence on TRIM, this can be important for both performance and NAND wear management. TRIM is often referred to as garbage collection, which is somewhat true. 

Q: What has Apple changed exactly in Yosemite?

Prior to 10.10, it was possible to edit KEXT files, effectively driver parameters, without raising any flags in the OS. 10.10.x adds another layer of security, which validates these KEXTs and causes a halt if one has been modified in an unauthorized way. Essentially 10.10 is validating KEXT authenticity now to prevent one that is modified from operating and causing unexpected consequences within the OS.

In the case of the TRIM hack — modifying the KEXT is the only way to enable TRIM for a non-Apple drive. A modified KEXT that is then seen as non-trusted results in 10.10 halting during startup. A workaround is a mode that disables security, but that’s not a great idea to begin with. Plus the changes are stored as a PRAM variable, so if the system PRAM is reset (as is sometimes necessary), the system stops booting again.

Q: Did third-party SSD makers get any warning of this change?

With one exception, no third-party SSD maker promoted use of/included a TRIM enabler hack for their drives.

Prior to 10.10, each time a new OS revision update came out, these TRIM hacks would be disabled and would require re-enabling. What the hack does is basically tell the OS that the non-Apple SSDs in the system are Apple drives so TRIM can be engaged with them.

As for warning, developer versions of 10.10 made the change clear early, and it was known that this method of non-approved driver modification was no longer going to be allowed.

Q: So Apple has never endorsed this practice?

Correct. Apple has never supported the TRIM hack.

Q: So, do all SSDs need TRIM?

In 2008, before Apple had any OS side TRIM control, we saw SSDs as a great new space for system acceleration. They were still expensive but for the performance they appeared to offer, the benefit was incredible. Only one problem – these SSDs would slow down and could do so quickly, depending on the usage profile when used in a Mac. Then we started working with the new Sandforce controller. This controller worked very differently and was designed to excel without the need for TRIM. While LSI/Sandforce based drives do support TRIM, in years of real-world testing we can confirm this processor effectively negates the need for such. Performance remains steady and true whether the drive is empty or 99.9 percent full and the NAND wear profile is the best hands down, and by wide margin, compared to its next closest rival even when it is using full TRIM support.

The only reason we entered the SSD space when we did was because by early 2010 we could offer a drive that simply didn’t need any OS side TRIM crutch. This was the right solution, and the best SSD solution we could offer our Mac customers where no slowdowns or other issues would occur in a TRIM free environment. Here was a drive that performed better than other drives even when it became possible to use the TRIM hack on a Mac following 10.7 Lion.

Today it’s fair to say that all drives have better internal garbage collection capabilities and that reduces dependence on OS side TRIM. Our LSI/Sandforce processor based SSDs provide higher and more consistent performance, without excessive NAND wear compared to most other processor-based drives on the market today, even those that still depend on the benefit from the OS carrying out TRIM commands.

Q: What does the future hold?

We are looking forward to larger and faster drives with LSI/Sandforce’s next generation processor next year — but in many cases, the current drives provide real-world performance that is still tops. These drives excel not just in benchmarks, but also in a real system load environment where the heavy lifting is done. One part of this is that the advanced internal block management capabilities of the processor can do what needs to be done without the kludge that TRIM from the OS can be.

Q: Other than installing Yosemite, is there a way for users to find out if their SSD uses TRIM and will have problems?

All SSDs can use TRIM. If users have installed a TRIM enabler to enable TRIM commands with their SSD, they will have problems when updating to Yosemite unless they remove the enabler.

Q: What do you think about the fix that involves disabling KEXT signing? Do you see any drawbacks to using utilities like Trim Enabler?

This effectively disables a security feature. But that aspect aside, this method of fix is accomplished by changing a variable that is stored in PRAM. If the PRAM gets reset, the fix is off until you fix it again. Also, it will be interesting to see how each update affects with respect to 10.10.1, 10.10.2, etc. 

Q: So the hack might not work in the future?


Q: What hardware would you suggest for someone wanting to upgrade an old Mac with an SSD but who wants to avoid the hassles of disabling KEXT?

Simple — any of our SSDs :)


Great for Mac and PC too, but the design from day one was focused on these being a solution for the Mac where TRIM initially wasn’t even supported at all under OS X for any SSDs (not even Apple’s), and later only by hack on non-Apple drives. These drives are not dependent on the crutch OS side TRIM offers and are better for it in general, especially so on the Mac where there is no need for any hack to ensure best performance and NAND life. 

Q: How difficult is it to upgrade a Mac from a hard drive to SSD, or upgrade an existing SSD?

Really very easy.

A relatively simple swap and then, for Mac users, the Apple Data Migration application sourcing from the original drive or restoration from Time Machine automates restoring your data to the new drive and then you are right where you left off and off to the races. The actual drive swap/installation process varies by system, but can be as few as 5-10 minutes to accomplish. We have a full library of DIY videos to assist all users, free to view, on the process of swapping a drive:


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