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.)
First, you need to find out what macOS’s internal representation of the disk is:
Plug the drive in and power it up if necessary.
Type the following and press Return:
In the resulting list, find the disk number associated with the unmounted volume. You may see multiple entries that start the same (as in the figure), such as
disk3s1, and so forth. The first part is all that’s needed.
Type the following and press return:
diskutil eject diskX
diskXwith the number of the disk, like
Power down the drive if it has a power switch. Disconnect it from the Mac in all cases.
Reconnect the drive and power it up if necessary. It should now appear on the desktop.
The Macworld reader who reported this problem and that the solution worked for some drives had to use an additional bit of troubleshooting to fix another. A background process called QuickLookSatellite, which manages some aspects of generating previews for QuickLook in the Finder and elsewhere, had stalled. You can force quit that process via Activity Monitor:
Launch Applications > Utilities > Activity Monitor.
In the find field in its upper-right corner, enter
Select each match that appears and click the X (Force Quit) button in the top-level corner, and confirm by clicking the Force Quit text button.
QuickLook will automatically restart any processes it needs, so there’s no need to launch the background process by itself.
This Mac 911 article is in response to a question submitted by Macworld reader Fiona.
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Source : Macworld