When you mount a drive in the Finder, you expect it to stay there. If you’ve found that your previously reliable external hard disk drive or SSD starts ejecting itself, trouble is obviously afoot.
The message “Disk Not Ejected Properly” usually appears when you unplug a cable or disconnect power to a drive without making sure the disk has unmounted from the Finder after selecting it and choosing File > Eject [Name] or clicking the Eject icon next to its name in the sidebar.
If happens other times, it’s never good. Some readers have found this problem is related to system software, and they go through the standard set of hardware parameter troubleshooting, such as resetting NVRAM and resetting SMC.
It’s worth trying, because it’s non-destructive. If unexpected unmountings continue, try to make a backup immediately. This may reveal other problems, but it’s important you have as current a copy of what’s on that drive as possible, as the hardware may be about to die—or the drive may at least be hard to access while you work through solutions.
Read more at MacWorld.com