What is TRIM and do I need to turn it on in OS X?


Some SSD drives are crashing in Yosemite do to an issue with something called ‘TRIM’.  Thankfully if you followed the method in my article entitled “How to speed up your mac with a Solid State SSD Drive” you will not have this problem. But if you have used an SSD that needs TRIM, read on to find out about the problem and how to fix it.

What is TRIM?

Trim is a command your computer gives to your SSD hard disk to tell it to delete a very small amount of data.

If you think of data as papers in a filing cabinet, to remove an empty page (if you were an old hard drive) you would need to take out an entire folder of papers, then  find and remove one page, and then put back the entire folder.  This is because spinning hard disks deal with huge chunks of data called blocks, which are like a folder, and they can’t deal with anything smaller than a block of data. They spin so fast and the blocks are so big that doing an entire folder at a time is the fastest way to do it.

With an SSD drive they are different and so it’s much faster to do it 1 page at a time. In fact it’s slow to deal with writing large chunks of data. TRIM is a command that allows an SSD drive to remove (delete) 1 page at a time instead of a whole folder at a time.


TRIM and no TRIM SSD’s

In order for TRIM to work the SSD must have ‘TRIM’ support, and the computer must have ‘TRIM’ support.

Some SDD’s are enabled to work with TRIM.  Others have chosen to deal with deleting data in a different way. They have their own ‘filing’ system. So the computer says ‘delete this page’ and the SSD does it automatically. It’s a bit like having your own personal secretary to do your filing. For those SSD’s it’s actually better to keep TRIM turned off and leaving it all to the secretary, so to speak.

For example this is what OWC say about TRIM:

If you have an OWC SSD, though, you don’t need TRIM. The SandForce controller in our SSDs takes care of this “garbage collection”… In fact, enabling TRIM could actually hurt the performance and reliability of your OWC SSD, rather than help it.

Apple and TRIM

Apple have not allowed TRIM support unless you have an Apple installed SSD Drive. That means if you want to install your own SSD into a mac, you have 2 options.

(1) Install an SSD that doesn’t need TRIM.  

Here are some SSD’s that work better without TRIM:

– any SSD with a Sandforce controller.

– All OWC Mercury Drives.  (read this)

– Crucial M4

– Samsung 840

(2) Use an SSD that needs TRIM but run a hack that turns on trim support in OS X.


Here are 2 such hacks:



Disaster in Yosemite!

Many people chose option 2, to use an SSD with TRIM and run the hack, but in Yosemite all TRIM enablers/hacks were disabled by Apple. Anyone who did this now found their SSD would not boot in Yosemite. Nasty move by Apple. This was a disaster! (Thankfully, in my SSD article I recommended to use drives that did not need a TRIM enabler.)

The Fix

#1 PREVENTION is the best cure: use a drive that doesn’t need TRIM.

#2 If you have a TRIM drive already, and use the TRIM hack, the writers of the TRIM enablers are trying to get around this.

#3 If you don’t yet have Yosemite, grab a non-TRIM SSD and copy your data across to it before you upgrade to Yosemite.

#4 if you have already upgraded to Yosemite and are now  getting a grey screen, go to this article and scroll down to the section called “Recovering from stop sign on boot screen.” It has some steps to help.


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