macOS alerts you when a remotely connected drive was disassembled before the operating system had a chance to clean up any loose ends on it. In some cases, you may not be able to mount a drive that was ejected early due to a power outage, shut down your computer abruptly, or unplug before macOS was ready.
A few Terminal commands can help in some cases, including solving a problem for a Macworld browser that had assembled three drives that could be mounted under Windows, but macOS refused to mount or let Disk Utility perform repairs. (This issue may affect drives formatted for Windows and macOS that are mounted more than HFS + or APFS formatted drives, but this is unclear.)
First you need to find out what macOS internal representation of the disk is:
Connect the drive and switch it on if necessary.
Start the terminal.
Enter the following and press Enter:
In the resulting list, find the disk number associated with the unassembled volume. You can see multiple entries starting the same (as in the figure), for example
disk3s1, and so on. The first part is all that is needed.
Enter the following and press return:
diskutil eject diskX
diskXwith the number on the disk, which
Turn off the drive if it has a power switch. Disconnect it from the Mac in all cases.
Reconnect the drive and turn it on if necessary. It should now appear on the desktop.
The Macworld browser that reported this issue and that the solution worked for some drives had to use an additional troubleshooter to fix another. A background process called QuickLookSatellite, which manages some aspects of generating QuickLook previews in Finder and elsewhere, had stopped. You can force to end the process via Activity Monitor:
Start Applications> Tools> Activity Monitor.
Enter the search field at the top right
Select each match displayed, click the X (Force Quit) button in the top level corner, and confirm by clicking the Force Quit text button.
QuickLook automatically starts all the processes it needs, so there is no need to start the background process by itself.
This Mac 911 article is an answer to a question from Macworld reader Fiona.
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