I have an iMac from 2011, Mac OS X 10.6.8, and an SDD in a FW800/USB3 case. Which interface can let me enable TRIM: USB2, USB3, FW800 or Thunderbolt ?
According to OS X Daily, TRIM is enabled for all Apple SSDs, but not for third party drives. The article claims that a third party utility, Trim Enabler, turns on TRIM for third party drives as well. More details can be found on the developer's page. Note that this utility isn't officially supported by Apple, so you should back up before installing the utility.
The interface of the external drive shouldn't matter for TRIM support (USB 2, FW800, or Thunderbolt). However, please note that Macs did not begin to natively support USB 3 until June 2012- with your 2011 iMac you'd be using USB 2 with one of those drives.
I realize this question is over 4 years old, but I'm adding a comment because this page was the first hit for a Google search on the subject and some of the comments are outdated whereas others were plain wrong.
Apple added native TRIM support in OS X 10.6.8 but only for Apple OEM SSDs. To get around that restriction, software such as Cindori's TRIM Enabler allowed TRIM to be used on 3rd party SSDs. Later, OS X 10.10 (Yosemite) introduced a security requirement called kext signing which prevented TRIM Enabler from working without completely turning off kext signing. Then in OS X 10.10.4, Apple introduced a “trimforce” command which enabled TRIM on 3rd party SSDs without turning off kext signing. (I have sources but Stack Exchange won't let me post more than 2 links)
Contrary to many of the comments here, the interface DOES matter. The vast majority of USB enclosures do not support TRIM, but allegedly there are some that support it such as the JMS 578 USB bridge controller (source: http://bbs.pceva.com.cn/thread-125643-1-1.html). All five of the USB 3 enclosures / docks I have support it. Firewire supposedly supports TRIM, but I haven't tested it. Most Thunderbolt enclosures should support TRIM, and I currently have TRIM enabled on a SanDisk Extreme SSD in an Akitio Thunderbolt 2 enclosure. I used the trimforce command in terminal to enable it.
In order to check if your enclosure supports TRIM, you’ll want to go to Apple > About This Mac > System Report > Hardware > SATA/SATA Express > and look for “TRIM Support” under the enclosure specs. If TRIM Support is not listed, your enclosure most likely does not support TRIM. If it is listed but says “No”, you can try turning TRIM on by using the trimforce command in terminal. Just open Terminal and type “sudo trimforce enable”. Once your computer restarts, check System Report again to see if TRIM Support changed to Yes.
Finally, TRIM does matter, even for modern SSDs with sophisticated garbage collection. I experienced this firsthand when my SanDisk SSD started slowing down as it got older before enabling TRIM on it. There’s a good, but somewhat outdated article about it here: http://arstechnica.com/gadgets/2015/04/ask-ars-my-ssd-does-garbage-collection-so-i-dont-need-trim-right/
Yes, you can enable TRIM on an external SSD. I own a Lacie Rugged USB/Thunderbolt 128 GB drive and I enabled it with TRIM Enabler. I am using it as a boot drive.
I do not know if it is smart to enable TRIM but I could not find any posts where it would stand that TRIM can harm your external SSD.
But for any case, I have multiple backups of my SSD drive.
I have a Crucial 960gb m500 ssd that I put into a G drive thunderbolt/usb chassis a couple of years ago for time machine and random manual file backup. It seems to have started slowing down recently with "preparing backup" msg that last for a couple of hours before doing the backup. I ran trim force with the drive attached via thunderbolt, My osx system report now says that trim is enabled for this device. I'm waiting and seeing if it makes a difference!