Your 27-inch iMac includes the Apple T2 Security Chip, which provides the following security features to protect what’s on your computer and prevent unauthorized software apps from loading during startup:
Secure storage: The storage drive of your 27-inch iMac is encrypted with hardware keys to provide advanced levels of security. In the event of a catastrophic failure, data recovery may not be possible, so you need to back up your files to an external source. See the Apple Support article About Startup Security Utility.
Note: In rare circumstances, such as a power failure during a macOS upgrade, your 27-inch iMac may become unresponsive and the firmware on the chip may need to be revived. See the Apple Support article Mac models with the Apple T2 Security Chip.