You Need to Do More Than Delete Files on an External Drive to Make Them Permanently Disappear

There’s a trick to thoroughly wiping all your data before you junk your old external solid state drive—or even computer.
MUO on MSN

This is what happens to a file when you delete it from your SSD

What this means is that the deleted file is not fully gone and can still be recovered. It's only when the data is overwritten ...
Ars Technica

Using terminal to erase external SSD hard drive

The disk in question is disk04 - a 128 GB SSD drive in an external USB enclosure. I booted the Macbook from a X.11 USB install drive and there was an internal SSD drive as well. a simple fdisk ...