I have an Orange Pi + 2 single board computer with the ability to connect an external hard drive via SATA.
Armbian is installed on the hard disk, the bootloader is in the onboard memory of the board.
Any command to turn off the device leads to the same result: the indicators on the board go out, it is no longer available via SSH, but the hard disk, any devices connected via USB continue to work, and the image via HDMI is also displayed on the screen.
How to completely turn off the device so that the hard drive stops correctly?
I believe that if you are capable of powering the device off using the operating system's menu, then it should be safe to cut the power off from the device itself. I have a raspberry pi 4 and have a little USB-C adapter that has a simple power switch which makes this concept very convenient for me. Cutting power to a turned-on, working device is very dangerous as it can damage critical files that could be being written. For this reason, it is probably smart to back up the device and also make sure that you are powering the device off through the operating system so that it can finish editing core files in case it needs to.