Enable trim on external SSD?

that command enable TRIM for Third-party SSD drives on your system. If you have an Apple Brand SSD as your boot drive, it will not be affected in any way -- TRIM is already on for that Drive.

To check use:

 menu >About this Mac > (System Report) > SATA (or whatever interface

Select the drive in the list

look for the Line: TRIM support Yes

KINGSTON SVP200S3240G:

Capacity: 240.06 GB (240,057,409,536 bytes)

Model: KINGSTON SVP200S3240G

Revision: 503ABBF0

Serial Number: 50026B722601F76F

Native Command Queuing: Yes

Queue Depth: 32

Removable Media: No

Detachable Drive: No

BSD Name: disk0

Medium Type: Solid State

TRIM Support: Yes

Bay Name: Bay 3

Partition Map Type: GPT (GUID Partition Table)

S.M.A.R.T. status: Verified

Volumes:

EFI:

Capacity: 209.7 MB (209,715,200 bytes)

BSD Name: disk0s1

Content: EFI

Volume UUID: 0E239BC6-F960-3107-89CF-1C97F78BB46B

X12-240:

Capacity: 239.2 GB (239,197,650,944 bytes)

Available: 139.81 GB (139,808,923,648 bytes)

Writable: Yes

File System: Journaled HFS+

BSD Name: disk0s2

Mount Point: /Volumes/X12-240

Content: Apple_HFS

Volume UUID: A7D14BFD-3696-3FD3-A5B9-2875B8CAEC52

Recovery HD:

Capacity: 650 MB (650,002,432 bytes)

BSD Name: disk0s3

Content: Apple_Boot

Volume UUID: 11E584FB-138F-30F5-B0D7-D1FB92798D87

use this Terminal command Apple provided in 10.10.4 and later:

sudo trimforce enable

This turns on TRIM for all drives on a System. The setting is stored as part of the running MacOS, and is honored in Safe Mode.

There are some anecdotes that suggest TRIM may not be enabled for certain External drives. Exactly which are allowed/not is not clear.

Older versions of Disk Utility would allow you to "erase free space" on a drive, which might help keep an SSD in better shape, but that GUI option has been removed.