I migrated my external hard drive from a USB enclosure to an OWC miniStack STX thunderbolt enclosure and started getting the "Drive not ejected" messages non-stop when the computer went to sleep. Computer is a MacBook Air M1. Things I tried that did not work: Boot into safe mode & reboot Reset PRAM In the end, what did work: Open a Terminal window and set the disk drive sleep time to a longer interval (in my case, from 10 minutes to 20): sudo pmset -c disksleep 20 (the -c option sets the power management settings for when the Mac is connected via a charger). Slightly shorter durations (e.g. 15 minutes might work as well, I've not tested that, but I suspect that you need to make the disksleep duration > displaysleep) You can verify your newly changed settings via: sudo pmset -g live Here's what mine are now set to: System-wide power settings: Currently in use:  standby              1  Sleep On Power Button 1  hibernatefile        /var/vm/sleepimage  powernap             1  networkoversleep     0  disksleep            20  sleep                1 (sleep prevented by sharingd, bluetoothd, powerd)  hibernatemode        3  ttyskeepawake        1  displaysleep         10  tcpkeepalive         1  lowpowermode         0  womp                 1