There are a few non-free GUIs that do that with a custom driver (thus without modifying system files) even on older Macs and systems:
Trim Enabler or
Disk Sensei [same developers].
More: The command works with internal drives only, I believe. If TRIM can be enabled on external drives, someone else will have to kick in with how.
notes: A Few Additional Notes About TRIM. TRIM isn't supported in external enclosures that use
USB or
FireWire as the method of connection to your Mac.
Thunderbolt enclosures with SSDs do support the use of TRIM.
Another thing on SSDs that I was thinking about: Would TRIM work/be functional on an encrypted disk? I presumed that free space on a disk is also encrypted (and thus never be marked as available space on the low level that TRIM will zero), but it turns out File Vault (File Vault 2 whole disk encryption) doesn’t work that way. The free space is not part of the vault, and TRIM will empty those blocks just as with an unencrypted disk. Encryption purists wouldn’t be happy about that, as even the amount of free space could be a marker, that real full disk encryption wouldn’t give away.
