Enable TRIM/discard on external SSD

First, find out whether your device already supports TRIM commands.

$ lsblk --discard

Non-zero values in the DISC-GRAN and DISC-MAX indicate support. If it looks like your external SSD doesn’t support trimming, then maybe it supports UNMAP which is equivalent (UNMAP is just in the SCSI command set vs TRIM which is in the ATA command set). Assuming your external drive is /dev/sda

# apt install sg3-utils
# sg_vpd -a /dev/sda | grep -i unmap

If the last command has Unmap command supported (LBPU): 1 it means the drive supports the UNMAP command. If it’s supported, and discard wasn’t supported, it’s likely the kernel didn’t detect the UNMAP support. You can verify it by reading /sys/block/sda/device/scsi_disk/0\:0\:0\:0/provisioning_mode

$ cat /sys/block/sda/device/scsi_disk/0\:0\:0\:0/provisioning_mode 
full

full means no support. As we know our device supports unmap we can manually instruct the kernel about it.

# echo "unmap" >/sys/block/sda/device/scsi_disk/0\:0\:0\:0/provisioning_mode 

Now, lsblk --discard should report that the drive supports trimming, and you can use fstrim to trim it.

Making the change permanent

The changes above are ephemeral and will be reverted once you disconnect the drive. If you want to automatically apply those changes whenever your external drive is connected, we need to use udev rules.

Add the following rule to udev under /etc/udev/rules.d/90-usb-discard.rules

ACTION=="add|change", ATTRS{idVendor}=="0b05", ATTRS{idProduct}=="1932", SUBSYSTEM=="scsi_disk", ATTR{provisioning_mode}="unmap"

Replace idVendor and idProduct above with the corresponding values for your device, as can be found in the output of lsusb.

Reload the udev rules using

# udevadm control --reload