macOS offers a number of drive encryption options. With a Mac with a T2 chip built in, the startup volume is always encrypted. On other Macs, enabling FileVault encrypts that volume. (On all Macs, FileVault provides additional protection for a powered-down Mac, too.) You can also select a drive in the Finder, Control-click it, and choose Encrypt “drive name”, setting a password of your choosing.
If you forget that password or were given or purchase a drive that’s encrypted, you might think you’re at a dead end. However, it’s not dire—so long as you don’t need the data on the drive. If you merely want to erase it, Disk Utility will oblige:
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Launch Applications > Utilities > Disk Utility.