TRIM is a command that an operating system sends to a solid state drive to indicate which blocks of data are no longer in use, for example after a file has been deleted. This lets the drive's internal controller clean up and prepare that space in advance, rather than only discovering it is free the next time something is written there.
Without TRIM, an SSD's performance can gradually decline as it fills with a mix of genuinely used and abandoned data blocks, since the drive has to do more work at write time to make room. TRIM support helps an SSD maintain more consistent speed over months and years of use.
TRIM support for an external USB SSD depends on the drive itself, the enclosure it is built into, and the operating system all supporting it end to end, which is not always guaranteed with basic external enclosures. See our external SSD guide for more detail.
