macOS alerts you when an externally connected drive was unmounted before the operating system had a chance to tidy up all the loose ends on it. In some cases, you may be unable to remount a drive ejected early due to a power outage, shutting a computer down abruptly, or pulling a plug before macOS was ready.
A few Terminal commands can help in at some cases, including solving a problem for one Macworld reader who had amassed three drives that could be mounted under Windows but macOS refused mount or allow Disk Utility to perform repairs on. (This problem may affect drives formatted for Windows and macOS mounting more than HFS+ or APFS formatted drives, but it’s unclear.)
