Secure erasing your external hard drive is very important if you wish to erase all data from the drive, but it can also be very dangerous because the data will most likely be irretrievable after the secure erase has been completed.
In order to securely erase your external hard drive:
1. Select the drive that you wish to erase on the left and click the “Secure Erase” utility.
2. Choose your settings for how you wish to perform the secure erase including: the strength (amount of passes), just the free space, and to initialize the drive for macOS after securely erasing.
3. Press the “Start” button on the bottom right; this will begin the Erasing process.
Note: If the secure erase is stopped anytime during the process, you risk the data being recoverable by someone using a data recovery tool.