As has been said MANY times before, cron doesn't create the same environment that you get when you login. And, there is nothing in your script that attempts to set variables that are needed in your script (in this case, cron is running your job with a default setting for PATH, and it appears that fstrim is not found on the default PATH) or fstrim needs something set in its environment that cron does not provide.

If you change:

Code:

    else echo "Trimming $1 $(fstrim -v $1)" >> $LOG

to:

Code:

    else echo "Trimming $1 $(/absolute/path/to/fstrim -v $1)" >> $LOG

where /absolute/path/to is the directory in which the fstrim utility is located on your system and fstrim doesn't depend on any environment variables being set to values not initialized as needed by cron, it might work. Otherwise, you need to set up your login environment in /etc/cron.hourly/trim using the initialization files your login shell invokes when you login, or explicitly determine exactly what is in your environment when your script works that is not in your environment when your script fails AND then be sure the needed environment is created in /etc/cron.hourly/trim before it invokes fstrim.