I have a new Pegasus 3 24TB drive, and a new Macbook Pro. Sometimes depsite being on, the Pegasus is not seen on Finder. When that happens, I also cannot swith the Pegasus off by depressing the button on the Pegasus. The only way to get the Mac to see the Pegasus, is to unplug the Pegasus' power cord, and then plug it back to restart. I run High Sierra on the Mac.
Normally, if I press the button on the Pegasus, it goes red and then switches off.