You are forgiven for thinking that using Disk Utility’s Erase function is secure enough to return or give away your external SSD disk.
However, all it does is just format the disk in the quickest way, sort off like a “wipe out the old data.”. Though, any recovery tool will still be able to get to your data.
If you do not, have an SSD disk, then Disk Utility provides Security Options when you use Erase. But what about