I ran into this issue 2x so I'm just documenting it for my future self, possibly others.
OS: Windows 10
After plugging in a USB drive nothing happened. In the partition manager, the disk appears offline. For some disks, the contextmenu provided an option Online to bring it online, then the drive was accessible. Other times, the option was grayed out.
There was a note in the partition manager (the app called Create and format hard disk partitions) saying:
the disk is offline because of a policy set by the administrator
I could make individual disks accessible via the
diskpart program but this was my girlfriend's machine so that was not an option as I'm not always there to help.
The following did a permanent fix.
Run diskpart and enter the following command:
Here is how it looked in action:
Now, interestingly, this did not fix the issue with the current drive I was playing with, when plugged in the same USB port as before. But it worked immediately when plugged in another USB port, plus, other devices that were not plugged in before worked immediately on all USB ports I tried.
Hope this helps somebody.