trimforce enable does not work I am using 2009 macbook pro and I wanted to upgrade the default hdd to a Samsung Evo 850. I have succeeded to transfer everything from a Time Machine backup, but when I wanted to enable the trim by sudo trimforce enable, then after a restart computer just shows the "crossed out circle".  I am running OSX 10.11.6, and AFAIK it should just work.
Am i missing something there?

What I did exactly is:


put a clean ssd into the computer
booted from USB install disk for El Capitan
restored the backup from the time machine
logged in successfullty to a newly setup osx as a user with admin rights,
saw in system information SATA suport "Trim support: no"
in Terminal typed sudo trimforce enable, after a minute computer reboot
crossed out circle shows (every 10 seconds it changes for approx 0.3 seconds for apple logo, but the computer does not boot)


am I missing something?

Edit. On page Crindori Disk sensei I found:

This is the easiest and safest way to enable Trim on OS X.
It is even better than using the new built in trimforce command,
since that requires 10.10.4 and may require disabling system security on El Cap.


Are they up to something with "disabling system security on El Cap". Should I still have done the kext-dev-mode=1 thing?
I'm using a Samsung 850 EVO in my MacBook Pro. If you prefer a method that uses the GUI, I recommend Trim Enabler (it's $9.99, works well, and supports the EVO 850):

You also asked why they mentioned that trimforce might not be safe. This article in their FAQ might answer your question: Why should I not use Terminal commands like trimforce?